What does this 5.1 mean for audios?

Hello in Netflix eg stands then with audio selection German 5.1 what does that mean? Is that different?

Multi-channel sound.

That you should only choose if you have a corresponding facility.

If you only have normal stereo speakers, you should choose 2.0.

5.1 stands for a multi-channel system that creates a surround sound with 5 speakers (3 front and 2 rear) and 1 woofer.

And if I use normal headphones

That's only 2 speakers in it. So stereo.