In plain English, grammar is the set of rules that guide and control the way you use words to make understandable sentences. When you're speaking or writing informally, you don't have to strictly stick to the rules, but in academic writing and other formal methods of communication, because you need to clear about what you trying to get across and want to create a good impression, good grammar is essential.
These pages aren't any kind of comprehensive listing of grammar rules, but hopefully give an introduction to the basics and let you know common issues that can arise when you're trying to write formally.
After looking at this section, you should have an idea of:
Image source 25 Jokes And Puns No Grammar Nerd Can Resist https://www.buzzfeed.com/weareteachers/25-jokes-and-puns-no-grammar-nerd-can-resist-h0xt