If someone is on trial, they are being tried in a court of law.
He is currently on trial accused of serious assault.
He will go on trial later this month charged with murder.
Two skinheads were put on trial for those murders last week.
See full dictionary entry for trial
on trial
phrase
If you say that someone or something is on trial, you mean that they are in a situation where people are observing them to see whether they succeed or fail.
The President will be drawn into a damaging battle in which his credentials will be on trial.