Keyword difficulty indicates how easy or difficult it will be to rank for a particular phrase or word. This makes it clear how much effort and resources are needed to optimize a page or site and whether there is rationality in this. By the complexity of the keywords, you can also do a superficial analysis of the level of competition in the niche and possible ways to easily bypass it, that is, rank for less difficult keywords.
You can view keyword difficulty in Ahrefs. The score ranges from 0 to 100, where 0 is the easiest and 100 is the hardest.