When people pass in the street, see a friend on the other side of the office etc.
They nod their head. It's an acknowledgement that they have seen you.
Women as well as men will do this.
Wro, this must be common only in wroclaw because this is a very popular custom in ireland, often also accompanied by people raising their eyebrows at you..... yet I've explained it to Polish people from all areas of Poland from Zakopane to Gdansk.
People get used to it, but at first they misunderstand and think they have been snubbed.