What do png mean?

If you're talking about image files, PNG is short for "portable network graphics" and is a file format similar to, but distinct from, the GIF standard. PNG was developed largely due to two problems with GIF:

GIF only supports 256 colors within a single image

GIF uses the (patented) Lempel-Ziv-Welch compresssion scheme

PNG avoids both of these pitfalls, supporting 24-bit color (about 16 million colors per image) and uses a different, nonpatented compression algorithm. The increased color depth is the main issue today, since the LZW patent expired about ten years ago anyway.