P's & Q's

Hi A!

Can't find what it means anywhere and now it's got me currious...

Any help appreciated.

And yes I'm not native English speaking.

To mind your Ps and Qs is to be careful; cautious. The Ps here are said to be pints and the Qs to be quarts. The publican "chalks up" or "puts on the slate" the drinks supplied to customers; they must be aware of how much they have drunk or their bills will be unexpectedly large.
An alternative view is that P derives from the French pied=foot and the Q comes from queue=tail(of a wig) and that the whole saying is based on 18th century court etiquette.