I'm going to gear my answer to more "novice" comic readers.
Traditionally, different Marvel comic books take place in the same universe. That means that something that happens in The Avengers book also happens in The X-Men book. For example, if Wolverine dies in The X-Men, you won't see him in The Avengers- unless he's brought back to life, of course! This "main" universe is referred to as Earth 616.
But this can be restrictive to writers- what if they want to reboot a character to have a more modern origin story? They solved this problem by setting their books in a different universe- they called this one the Ultimate universe.
So you've got two groups of books set in two different universes. (There are actually many more universes, but these are the main two.)
However, this can be confusing for new readers: if you want to start reading Spider-Man, you first have to choose which universe you want to read!
So in 2015, Marvel had a big event called Secret Wars. This event took place across every book in every universe, where different versions of characters met each other and new characters were invented (you might have heard of Spider-Gwen).
All New, All Different
At the end of the Secret Wars event, the 616 universe and the Ultimate universe were combined into a single universe- the All New, All Different universe, consisting of characters from both universes, as well as some new characters.
As a result of this rebranding, Marvel started a bunch of new series. But they also wanted to keep around some existing characters (like Squirrel Girl) who already had books coming out. So they started renumbering these books from issue 1 again, to make it more obvious that they were members of the All New, All Different universe.
What does this mean for what you should read?
Comic books are generally written in arcs, which you can think of as seasons of a TV show. An arc consists of one main storyline that then concludes, and a new arc is started. Arcs usually consist of 3-6 monthly issues, which are usually then collected into trade paperbacks for people who prefer reading them all at once instead of waiting a month between issues.
The renumbering of Squirrel Girl happened between the first and second arc. You can think of the first 8 issues as "season 1" of Squirrel Girl. In between season 1 and season 2, Secret Wars happened. Then the numbering starts over at issue 1, and season 2 of Squirrel Girl starts.
In other words, the renumbering is not a reboot, but they split it up between "seasons" to make it easier for new readers to get on board, since that was the whole point of Secret Wars to begin with.
So you can read from the very beginning (I'd recommend that, since it's a great book), or you can start at season 2 with the second first issue- they explain pretty much everything you need to know. You don't need to read any Secret Wars books or any other series to understand what's going on in Squirrel Girl.