"Commitment to a team is essential" sound like the kind of thing you'd see in an advertisement for a job opening. It would help if you gave some context for where you found the phrase.
Literally, it is stating that in order to be a part of the team you must be committed to that team. As the old saying goes, "a team is only as strong as its weakest link." Unless all members are committed to the team those members will act in ways that are in there individual best interest rather than what is in the interest of the group. Imagine, for example, if you are part of the basketball team and half the guys (or girls) don't show up for a game because there's a party going on that night...they would not be committed to the team and it would fail. Now imagine that everyone showed up to all the practices and games even when it wasn't convenient for them. That shows real commitment and increases the chances of success.