The defence barrister represents the person accused of the crime.
The defence do not have to prove that the accused is innocent, just that the prosecution's arguments are not good enough.
The barrister points out to the jury what they think is wrong with the prosecution's evidence. They will try to show that there are other possible explanations. Their arguments are called the case for the defence.
A barrister is a lawyer who specialises in appearing in court. Solicitors can also put forward the case in court.