The obvious way a dictionary can help you is when you want to look up a meaning of a word - if you don't know/aren't sure what a particular word means, you can look it up in a dictionary and then you should understand.
Another way a dictionary may help you is provide you with a translation of that word - for example, if you want to see what the Spanish word mariposa means in English, you will probably go look it up in a dictionary to understand the meaning.
Additionally, there are specific dictionaries, such as thesauri, where you can find synonyms or antonyms of particular words - so words which either have the same meaning, or the opposite meaning of the word you are looking for.