This set phrase is often used to remind team members that they have to work together, and not be selfish.
It's based on a pun: when you spell the word "team", you don't use the letter "I'. On the other hand, the word "I" means "me". So this phrase means:
- The team as a whole is important, not the individual members.
- You don't use the letter "I" when you spell "team".
"There's no 'I' in 'team'" is very well-known. It's used a lot in team sports like soccer, basketball, etc. Most people are actually tired of hearing it, so don't be surprised if your teammates ignore you when you say this!