I have just started reading about fitness landscapes and I couldn't understand what the peak means. What does the peak mean?
It's just the ordinary English-language meaning of the word:
[...] 4 a (1) : the top of a hill or mountain ending in a point
See, for example, Wikipedia:
Fitness landscapes are often conceived of as ranges of mountains. There exist local peaks (points from which all paths are downhill, i.e. to lower fitness) and valleys (regions from which many paths lead uphill).