Strategies to plan research and make the maximum out of it
Since scientific and math writers are writing for experts, and in fact specialists, it can be very difficult to read papers as you are getting started. Specialists share a mindset that implies that they can leave a lot unsaid. The author of a paper is likely very familiar with all of the papers cited, and you are not. This makes it especially difficult.
To understand a given paper it may be necessary to review many of the referenced papers first. At least the abstracts of those papers. It might be necessary to become familiar with some terminology that you aren't used to. Wikipedia can be a big help in many fields, actually.
But if you are starting out as a masters student then your education up to now has been mostly as a generalist, not a specialist. Don't get overly discouraged, but it takes a lot of work to build up your knowledge base to the point that you can understand a paper.
Dissertations are usually a bit easier, since candidates are expected to be a bit more complete than is true for published articles that depend on earlier work.
It is probably easier now than when I needed to spend hours in the math library, searching for referenced papers and then the papers referenced in those, etc, until I got some idea about what was being said. Now you can find a lot of those older papers online. The understanding hasn't gotten easier, of course.
But eventually, you grasp the specialized language and also the way of thinking of experts in your field. Lots of hours.
If you can ask questions of your advisor or others you can shorten the understanding process quite a bit.
As you read, take a lot of notes. Write down key ideas, of course, but also write down questions you still have. Then try to answer those questions either with more reading or by asking of others. If you find an answer in written work, it is good to write down the citation. You may want it later.
An effective researcher will also write down questions that aren't answered in the paper being read. This gives you an opportunity to extend the work.
Think of the knowledge you need as a tree that you need to explore. You need to go both wide and deep to acquire the understanding.