treiben means "to float" or "to drift". It paints a picture of something being carried along by a current, like a boat on water.
In the song, Wincent Weiss sings "Wir lassen uns treiben" (We let ourselves drift). This beautiful phrase is used metaphorically to describe letting go of all worries and plans, simply enjoying the moment and seeing where life takes them. It perfectly captures the feeling of freedom and spontaneity shared with a good friend.
Have you ever met up with an old friend after a long time and it felt like you just saw them yesterday? That's exactly the feeling Wincent Weiss sings about in "Hier Mit Dir". He describes how most of his friends have moved away to big cities like "Hamburg, München oder Berlin" (Hamburg, Munich or Berlin), so they don't get to see each other very often. But when they finally do, it's like "als wäre gar keine Zeit vergangen" (as if no time has passed at all).
All the stress of everyday life just disappears, and the singer feels pure joy. The song's main idea is simple and powerful: being with the people you love is all that matters. He says it's "so verdammt leicht" (so damn easy) when he's with his friends. This feeling leads him to the beautiful conclusion in the chorus: "Hier mit dir / Das ist der beste Ort der Welt" (Here with you / This is the best place in the world).