This summary is not the rule. The rule text controls.
If someone is arrested for a crime that allows bail, but they can't pay it or aren't released some other way, they may end up sitting in jail while waiting for their case to be resolved. If that person is later convicted and the court orders them to pay a fine, this rule gives them credit for the time they already spent locked up.
The credit works out to $5.00 for each day they were held before the conviction. This amount gets subtracted from the fine they owe. So if someone spent 10 days in jail before their case was decided, they would get $50.00 taken off their fine.
There's a limit, though. The credit can never be more than the fine itself. So if someone's fine is only $30.00, they can't use jail time credit to get money back or wipe out other costs. The credit just reduces the fine down to zero at most.