What does the opportunity record set "I am on the team" represent?

Record sets are used to represent your data set, or the data you've access to. A record represents a single opportunity and a record set represents multiple opportunities. Opportunities have inherent data security on them that depends on your membership in the sales team and territory hierarchy. The I am on the team record set means that the signed in user is a member of the opportunity team.

For example, if you're searching for opportunities, then the record set I am on the Team means that the signed-in user is part of the opportunity team for all the opportunity records returned.