It would mean to flirt, but in a way that is too obvious.

"overt" means it's clear for everyone to see. It's not hidden. So if it's a girl, maybe she was looking at him with seductive eyes while biting her bottom lip, sliding her foot up his shin under the table. Something with clear signals!

Thank you very much. I still wanted to know. :-)

Since you asked this over a year ago, I don't know if you still need an answer, but: To "flirt too overtly" means that someone is flirting too much, and too openly. It implies that they lack subtly, they're "trying too hard" and they appear to want attention