"Feel free to flex" or some variant thereof is a common expression used by Lars toward guests.
The term flex in modern slang refers to bragging about one's triumphs and/or qualities in a manner that is intended to impress. Often, but not always, the term has a negative connotation if the bragging is perceived as awkward or unnecessary.
Occasionally a guest will walk back their comments when they feel they have bragged too much. Lars will offer this in return. Ostensibly, as this is a talk show episode that exists to learn about a notable person, it is not inappropriate for said guest to talk about their accomplishments; some "flexing" is fine.