A person gets a petty theft charge when they steal something of very low value.
Usually the value of the item or whatever was stolen has to be under $500.00 for the stolen property to be considered petty theft.
Petty theft in most places is a misdemeanor which means that you likely get probation and or a fine and maybe a few months to a year in jail depending on the value of the stuff you stole.
However in some places petty theft is looked at differently and is charged as a felony no matter the amount of value of the property you stole.
If the petty theft is your first offense then you might or might not get arrested and you may just get a summons to appear in court.
But it can depend on the state and their laws as I've heard of a few people who got arrested for petty theft but they were able to bond out for around $500.00 but they got the money back when they appeared in court.