If a police officer says you're under arrest and you then try to resist getting arrested and fight the police during the arrest then you'll be tackled to the ground by other officers and arrested anyway.
Then when you get to jail you'll be charged with resisting arrest which makes the case harder on you with the additional charges.
If you're under arrest there's no use in fighting the cops because you're not gonna win the fight and will be under arrest and in the back seat of the police car and on your way to jail anyway.
If you try to fight the police you'll either be tazed or even have pepper spray sprayed in your face which burns and you'll be wishing you just put your hands behind your back and went peacefully with the police.
Resisting arrest can get you 6 months to a year in jail and your bail will likely be set higher because it shows you're a risk and may not show up to court.