Fadi,
Team buy in means that the team members all agree and support a project's vision and objectives.
Here is a short article on this topic for you to read:
kevinsuttonbasketball.wordpress.com/2010...in-within-your-team/
This article is written by a basketball coach, because buy in is a big part of a sports team as well. I like the last 2 statements in his article because they really explain how "deep" buy in can go and what it can do for a team.