What is the main idea of osmosis?

Could you possibly enlighten me on the core concept of osmosis? I've been trying to grasp it but it seems quite intricate. Is it related to the movement of molecules across membranes? Or is it more about the balance of concentrations? I'm really curious to know how this process works in biological systems and how it might apply to other areas as well. Could you explain it in a way that's easy to understand? I'd really appreciate it if you could break it down for me. Thank you so much in advance!