You need Forge 1.12.2 to install mods designed for Forge, which is many of them.
Forge is a "modloader". It's a kind of mod that does almost nothing but load other mods into the game and provide support to them.
You can mod Minecraft "directly", but then it's hard to run multiple mods, because if any two mods do something basic (like add blocks to the game) in a way that conflicts, then there's a good chance that one mod will break another. Early Minecraft mods tended to be "direct" mods. Modloaders are "direct" mods by necessity.
Using a modloader has the advantage that mods that are loaded through the modloader don't need to directly change the game's code, so there's much less chance of any two mods breaking each other—plus it's easier to develop mods in the first place because there are nice easy ways to inject game content that don't involve endless reverse-engineering of the game code.
Other modloaders exist. In particular, the Fabric modloaders has been gaining in popularity as an alternative to Forge that's lighter-weight and uses more elegant ways to integrate code. Unfortunately, Forge and Fabric aren't compatible, and can't be run simultaneously. There's a project to attempt to support Forge mods on Fabric with a compatibility layer, but it's not quite ready yet.
You need Forge for Forge mods Forge is a modloader It's possible but less pleasant to use mods without a modloader if they're designed not to use one—but it's much clunkier There are other modloaders out there, particularly Fabric.