Many people in the Witchlands are witches - folks born with magic. Most have some sort of elemental magic (water, wind, fire, earth), although some have powers based in the "aether" or the "void". Our two protagonists are a Truthwitch - she can tell if someone is telling the truth or not, and this is super rare/desired in this mythology - and a Threadwitch, which is someone who can see connections between people. At the start of the book, they upset a very powerful and rare Bloodwitch, these witches can control other peoples' blood, and spend the rest of the book on the run from him and other nefarious foes out to exploit the Truthwitch's rare powers.
This is a perfectly good fantasy series opener. It's got a very conventional system of magic (elemental magics aren't exactly a new concept), and while one of the main characters was one of those magical beings that men just die over, which, ick, I did enjoy the two main characters. Their strong female friendship was at the heart of the story, and I think that's great modelling for young women. The magic wasn't inventive, but there were a few fun new critters (sea foxes!) that I really enjoyed reading about. Fairly standard fantasy fare, but a solid first outing. I'd check out the sequel, and would expect the series to improve going forward. For the most part, I liked it.