A more thorough explanation:

Effective assistance of counsel is a right that criminal defendants have under the Sixth Amendment. This means that when someone is accused of a crime, they have the right to be represented by a lawyer who will help them defend themselves in court. The lawyer must make decisions about how to defend their client without interference from the government.

If a defendant's lawyer does not provide them with adequate legal assistance, then the assistance of counsel is not considered effective. The Supreme Court has said that the lawyer's performance must be objectively reasonable given the circumstances of the case. If a defendant thinks that their lawyer did not provide effective assistance, they can raise the issue in court.

To prove that their lawyer was ineffective, a defendant must show two things. First, they must show that the lawyer's assistance was unreasonable. Second, they must show that if the lawyer had not made those errors, there is a reasonable chance that the outcome of the trial would have been different.

For example, if a defendant's lawyer did not call any witnesses to testify on their behalf, and there were witnesses who could have helped the defendant's case, then the defendant might be able to argue that their lawyer was ineffective. However, if the defendant's lawyer made a mistake that did not affect the outcome of the trial, then the defendant would not be able to argue that their lawyer was ineffective.

The purpose of the right to effective assistance of counsel is to make sure that defendants have a fair trial. If a defendant does not have a lawyer who is providing effective assistance, then they might not be able to defend themselves properly. This could lead to an unfair trial and an unjust outcome.