What's the meaning of flex in that case?

Solution 1:

From the first still image (I didn't play the video) I guess it is given in the Urban Dictionary as

Showing off your valuables in a non-humble way.

You might like to bookmark this link to OneLook so you can research your questions. I looked in Urban Dictionary first, as I guessed it was street talk but had not come across it before.

Solution 2:

Flex can be seen as a way to show off your values or strength in something. A common expression is " to flex one´s muscles". You flex your muscles to intimidate someone somehow.

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