Such as the "amount of connecting vetices", so on and so forth. As well as the "equal", "greater", and "less".
They give more information if you let your mouse hover over the option. There is also the blender documents: https://docs.blender.org/manual/en/latest/modeling/meshes/selecting/similar.html#select-similar
I didn't dive into it any deeper than this, but if you're having trouble with any part of it you can also ask, and we can help explain how it works. For the most part though, they're fairly self-explanatory.
For example, Amount of Connecting Edges counts up how many edges the vertex is connected to other vertices with and selects all other vertices that also share the same number of connecting edges. The Greater and Less options allow you to select all vertices that have more or fewer connecting edges, respectively. So, if I wanted to select all vertices with more than four connections, I could select one that has four connections, use Amount of Connecting Edges, and then choose Greater.
The options in the Select Similar menu change depending on whether you're in vertex, edge, or face select mode, so there are quite a lot of them, but they all do the same thing - search through every other component and select the ones that match the criteria.
