This is a sentence that can be really useful. You are learning spanish and when you read a text could be very clear and easy to understand, but another thing is when some spanish-speaker is talking to you. They usually speak way too fast and probably you could make sense of what they mean if they repeat again. Then you must ask:
Puede repetir por favor?
To that question you can add 'I don't understand...':
'No le entiendo, puede repetir por favor?'.