What does git [master +2 ~1 -0 !] mean?

This is not an error, just some helpful information.

I suppose you are using Github For Windows - it uses posh-git by default, and the prompt is explained in the README of posh-git itself:

+ = Added files~ = Modified files- = Removed files! = Conflicted files

Go through the README and you can see how it relates to the git status output.

That prompt probably meant:
In your current git branch - master, you had added two files +2, modified one ~1 and removed zero -0.
! signifies you are currently at the root of the repository. Check this comment for more info.

Not sure which shell you are using - But you may want to check the PROMPT and PS1 environment variables to find what those items mean in your prompt.

The prompt got reset to just [master] - once you had committed all the changes, as you mentioned in your questions.

PROMPT environment variable is the one used by the windows shell cmd, and PS1 is used by unixy shells - like bash.

Edit: Totally missed the powershell tag you put in there - I'm not a windows guy - but looks like in powershell you create a prompt function - check this link: http://www.johndcook.com/blog/2008/05/12/customizing-the-powershell-command-prompt/