First of all reassure your child, if they could all understand everything straight away there would be no need for schools and teachers! It is also important for them to remember that if they were in school they would be able to see that it is not the case that they are the only one who can’t do it.
On a practical note these are the options available:
If they are still unable to complete a piece of work they should still turn in what they have been able to complete. This way the teacher will know that they (and others in the class) need further support for that topic.