Grammar lessons can be quite challenging. If you are a native English person you may not even be aware that there are nouns, verbs, adjectives and so on - you just use them without knowing (or caring) how the language is structured.
Grammar forms the set of rules for the English language and dictates how sentences and paragraphs should be constructed and also classifies all words into categories.
Grammar lessons can teach you about the structure of the language. If you are at school or writing is part of your job, or even if you send emails to other people or companies it is very important to have at least a basic grasp of grammar.
To learn grammar can be hard. The English language has a particularly complex structure with lots of spurious rules. If you don't have a natural flair for languages you may find that using a private English tutor is a good way to improve your grammar. A private English tutor can really make a difference to your understanding as they can concentrate on specific areas that you are struggling with.