What to do if I don't understand the related work?

The thing that strikes me most about your question is "the topic and goal are the same as mine". If this is the case, then those papers are intimately related to yours, and you will want to understand them for your own good. Granted you might not have the time - it is a Masters thesis after all. However, you'll still want to compare and contrast their results against yours. After all, a big question to answer is "is my method better than theirs?" and you cannot answer that question without knowing how the methods stack up.

You don't have to fully understand another work to write about it, but you might not be able to go into much detail: for example "Alice and Bob (2019) have also studied this problem using [method], but they ran into [issue] which we do not have" is perfectly fine. Nonetheless, with ten intimately related papers I think you will want to discuss this with your supervisor. That's a lot of related work, and your professor might already be aware of some/most of them. He might give you direction on which papers to write about, which to read in more detail, and so on.

One of the skills that you have to learn during your masters is reading papers and previous work done in this particular area. So I suggest you put extra effort to understand them. This will benefit you a lot and will definitely boost the quality of your master's thesis.

Also, learning about these previous papers might give you an idea to improve them and build upon them, and then you have your own paper published! This is how it actually goes in the research world. GoodLuck!

Can you perhaps mention the other research without endorsing it?

Previous work on the subject of frabishes was published by Smith and Jones [2012]. They claim to evaluate all frabishes using methods different than the methods we are using here.