When you read an English book, you often come across new words. You might try to guess what the words mean from the words you know, but when you cannot guess the meanings of new words at all, what should you do?
You have a good friend then. From him you can learn what a word means, how to pronounce a word, how to use a word, and so on. This friend can always be with you, and you can always ask him to help you. Do you know who this friend is?
It is a dictionary. You can learn some important things about words from it, so you must know how to use it.
come across 碰到 guess 猜测