This happens sometimes, and it is important for a client to tell his/her lawyer that they don't understand. A big part of the lawyer's job is to explain things and help the client understand. A client should not be shy about asking as many questions as necessary until they have a firm grasp of everything. Like my teachers in grade school said: "the only stupid question is the one that is never asked."
Having said that, sometimes if a person never understands what is going on, a psychological examination for competency may be in order. The lawyer should be watching for that issue and take the appropriate steps when it arises.
If it is simply a communication issue between the defendant and his lawyer, the defendant needs to inform his lawyer that he does not understand and needs things explained better. If the questions can not be explained in a way that the defendant can understand, or the communication issue persists, then the defendant should possibly try to find a lawyer that he is comfortable with and with whom he can communicate openly and understand.
Set an appointment in the lawyer's office to discuss the issues. Tell the lawyer about the areas you are not understanding and ask for clarification. If you cannot get satisfaction, then you should consult with another lawyer about taking over the case. You should be able to understand what is going on with your case.