Things are general assemblies in the Germanic world.
At PoD, their nature had already degenerated to a great extent among those Germanic peoples who lived close to the border of the Roman Empire. A military aristocracy had established itself among the Goths, the Quadi, the Markomanni, the Gepids, the Franks and others, and it was these aristocrats who took the decisions, even if the rest of the free adult males still attended Things. Farther in the North, Things still had their full function: disputes among clans were settled, military leaders were elected, general rules were repeated, clarified and sometimes mildly modified or specified.
In OTL, Things - and with them, Germanic peasants and other commoners - gradually lost their power from South to North. Kings became hereditary, laws were codified, jurisdiction was attributed to the king´s judges, and regional administration was carried out by feudal lords.
In this timeline, the Germanic world is comparatively more peaceful (for a lot of reasons, see Developments in Slavia and Germania Magna 251-750 CE for more details), and the influential Roman model is not one of absolutist autocracy and landowning elites ruling over an indentured colonate. Instead, the influential Roman model is that of local and federal democracy in comitia civitatum, which bear a lot of similarities to Things, in which a free peasantry enjoys power, influence and wealth in its co-operatives (societates liberorum).
As a result, Things continue to exist and have been gradually adapted to historical change. They are still the main and fundamental political institution in all Germanic countries.
Over time, Things have changed their functions. In prehistorical times, their role was mostly a judicial one, one of electing leaders and a ritual one. Jurisdiction has become attributed to permanent judges, assisted by juries drawn by lot, since laws were codified from the 6th century (Alemannia) to the 13th century (Aspeland). In exchange, Things became legislative bodies, where traditional law was collected and pinned down into codices, then modified, until in modern society, tens of new issues of legislation are decreed every year. Things still elect people with special functions - but today it`s not ritual kings, nor warlords, but "balivs" (from Latin "baiulivus" = governor), who are in charge of local and regional administration and are elected for periods of usually three years, their vicebalivs (who are in charge of e.g. the schools, or public transportation, or public utilities, or the police etc.) and the treasurer, and delegates to federal or national legislative councils. Ritual roles have been shed, too, as a major step in the secularisation of the Germanic states, which occurred as early as the 7th century in Southern Norway`s Egðirþing and Gulaþing, and as late as the 19th century in Sweden.
Things have also changed their composition. In the hierarchical societies of the late Iron Age, wealthy nobles had extensive powers, they usually brought "their" freemen and half-free along to vote with them, while slaves (thralls) and women, thus a majority of the adult population, were not allowed to participate. Slavery was abolished in the Suebic and Frankish countries between the 5th and 6th centuries, in Burgundy, Saxony and Sweden as late as the 12th century. Women were allowed to participate in Things only from the 15th century onwards (Alemannia was the earliest in 1401, followed by Franconia in 1403, while Saxon and Swedish women had to wait until 1542 resp. 1568 to obtain voting rights).
Historically, Things had a decidedly rural background, in contrast to the Veches of the Slavic world, which were general assemblies of the townfolk, or to Roman comitia civitatum, where town- and countryfolk assembled together. Today, Things have become assemblies held both in large cities and towns as well as in the countryside, where small towns, villages and hamlets exist.
Procedurally, Things have copied many of the practical innovations from the Roman world, e.g. conscriptio.
While there are many similarities between Things and Comitia Civitatum today, there are also a few important differences: for example, they are usually monolingual, the persons who lead the discussion ("lawspeakers") are regarded with great veneration and have much greater procedural powers than the censorial quaestors who call together a Roman comitium civitatis, and factions are less important, especially in smaller Things, than in the Roman world.