Repetition means to use something again. In the case of poetry it is to use a word, phrase or structure again. It is especially useful for emphasising the importance of something, the passing of time, or intense emotions. You can use repetition in your poems to ensure your message 'hits home' for the reader.
Whilst we all know Dr. Seuss books contain wonderful rhyme, they also contain great examples of repetition. Green Eggs and Ham is full of repetitive questions and statements that show just how exasperating the situation is. If you want to show frustration in your poem, how could you use repetition to reinforce your emotion?