tModLoader is an API for Terraria that provides a way to create and load mods without having to work with Terraria's source code directly. This means you can easily make mods that are compatible with other mods and save yourself the trouble of having to decompile then recompile Terraria.exe. This also means players can use multiple mods at the same time without having to worry about compatibility issues. Traditionally most mods (standalone mods) that don't a sort of API like tModLoader to handle them, will be incompatible with each other. tModLoader is made to work for Terraria version 1.3 and above.