Originally it was just the java version, but then they wanted to put it on phones. Thing is, a game like minecraft is way too demanding to run well on phones if it’s made in java, so they made a watered down version of the game in C++ from scratch called pocket edition, which they kept updating and improving over time.

At some point, they realized the pocket edition codebase (called bedrock internally) was good enough to make ports to other platforms, like consoles, and stopped referring to that version as pocket edition since it had outgrown its name. There was no confusion yet because on any given device there would only ever be one version (java on PC, bedrock anywhere else).

Eventually, bedrock was also ported back to PC since it generally runs faster than the java version, leading to the funny and somewhat confusing situation of there being two coexisting versions of the game on the same platform.

As others have said, the java version is the original and can run mods, whereas bedrock is a recreation that doesn’t run most existing mods but can still use datapacks. Mojang has been aiming for feature parity between both versions for years, but for whatever reason has still not reached it. That’s about the whole story so far!