The training program is essentially for learning the Bloomberg stack and the primary languages used during development. There are two tracks: one is short-form training (3 months, for CS/IS students) and long-form training (4 months, for people who weren't CS/IS students, have little experience in software, or want a slightly longer program for any reason).

The training is not optional, everyone has to go through it. If you know C++ and Javascript, you shouldn't have any difficulties whatsoever. Regardless, they will be teaching you these languages so you don't need any prior expertise, which is good. You'll also learn a lot of their standard stack, like their database, middleware, UI client, and communications, essentially the entire tech ecosystem. You'll have basic finance courses (which is cool) and will have many opportunities to interact with engineers in different teams. You'll learn how to use the Terminal. There'll be graded and ungraded tests/assignments between classes, culminating in a final team project which will actually be used in production. There's also a fancy graduation party at the end.

Unless you're a really bad programmer, you shouldn't have any problems with the program; it's a lot of fun. The only caveats are that some of the tech is antiquated, and you might end up in a team which doesn't use the standard stack at all, so spending 3-4 full months on it is not exactly a good use of your time. This doesn't mean that it all goes waste; you still have to interact with the main ecosystem one way or the another; plus the course content is awesome regardless of your knowledge on the topic.