What does a bandana in the back pocket symbolize for gay men?

This is called the "hanky code" and was more common in the 1970s and 1980s when most gay people were closeted. It is a code for gay men to let other gay men know what what they are interested in. The code has to do with the color of the bandana and which pocket it's in.

It still exists, but it's not very common anymore.

Hanky Codes