"Sort of" is colloquial and means essentially the same thing as "kind of" (I can kind of understand it), "a bit" (I can understand it a bit) and similar.
If someone says that they "sort of" understand something it means that they understand some of it but not all, or have a vague understanding but are lacking the details, or have only a basic understanding of a more advanced subject.
For example I could have this conversation with a colleague: - let's assume we both received the same email from Bob:
Me: Hey, do you understand this email from Bob?
Colleague: I can sort of understand it [= "I understand some/most of it", or something like that], but I don't know what Bob actually wants us to do.
I sort of cleaned out the cupboard/closet, but there's actually a lot of trash still in there.
You will often (especially, in my experience, in American English) come across these being expressed as "sorta" (sort of) "kinda" (kind of), even in writing (informal only). In fact "kinda-sorta" is now its own slang term! (see e.g. this answer).
Oh, and a similar usage: I have found myself saying things like:
I can sort of understand her point, but I actually don't agree that everyone should sign up with this website (or whatever) because XYZ [in relation to a contrary point of view]
I was sort of [= a bit] expecting to receive that letter today, actually!