Punctuation is a series of marks which can be used to separate words and groups of words to make the intended meaning clear and easy to follow. It can be used to emphasise certain words and phrases, and to distinguish between major and minor ideas.
Skilful punctuation is the key to good sentence construction and therefore to clear expression. It is much more important to understand how punctuation works, by studying examples here and elsewhere, than to memorise long lists of all uses of all punctuation marks.
Bad punctuation can have serious consequences, leading to difficulties in understanding and even to complete misunderstandings.