Reading a math textbook is often very difficult. This is partly because math textbooks are somewhat unlike other textbooks. In particular, they are much denser, and of course contain many symbols and formulas. In fact, mathematicians take pride in being “terse”, that is, effectively concise, which basically means putting a large amount of information is packed into a few words.
For other reading classes such as English or history, it is suggested to read quickly and skim through the textbook material, and to understand the material broadly to obtain the main ideas.
However, this is a poor strategy for math textbooks, because math textbooks are written to be concise, and so almost every point and detail is important to understand the concepts effectively.
On the other hand, skimming can still be done as a first read, to get the general idea of a chapter, to probe for questions, etc. before doing a second careful and slow reading.