"It's coming together" means "it's starting to be successful" or "it's getting closer to completion". 

Use this phrase to talk about a project. It can be one of many kinds of projects:

• a school assignment
• a construction project
• a website that you're building
• a novel that you're writing

You can also say "___ is starting to come together" to talk about a project that's a little further away from being finished.

This is a good general-use phrase; it's neither very formal nor very casual.