When somebody goes to court and they have a lawyer to help them, that lawyer has to do a really good job making sure the person's rights are protected and that they get a fair trial. But sometimes, the lawyer doesn't do a good enough job or doesn't do things that could have helped the person win the case. When that happens, it's called "ineffective assistance of counsel."
It's like when you have a really important task to do, like clean your room or get dressed for school, and you ask your older brother or sister to help you. If they do a bad job and don't help you get everything done that you need to do, you might get in trouble or be late for school. That's kind of what happens when a lawyer doesn't do a good job helping someone in court.
If someone thinks their lawyer did a bad job and it affected the outcome of their case, they can ask a judge to look at what happened and decide if the lawyer did a good enough job. If the judge agrees that the lawyer did a bad job, the person might get a chance to try the case again with a new lawyer. That's like getting a second chance to clean your room or get dressed for school – but this time, with a better helper!