Resisting arrest is just what it sounds like. If the police have probable cause to arrest you and you resist or delay them in some way, they can charge you with the Misdemeanor of Resisting Arrest. Some examples are running away from the police or providing the police with a false ID (which can then also add another Misdemeanor charge of Criminal Impersonation), refusing to allow yourself to be handcuffed, refusing to get out of your vehicle, motoring away from a police stop or refusing to comply with a lawful police order.
Be aware that officers may threaten to arrest you if you refuse to comply with a search request or refuse to answer their questions, however, don't get tricked. You still have the right to refuse a police search by saying ("Officer, I do not consent to any searches of me or my vehicle") and refuse to answer questions without a lawyer present. ("I wish to remain silent. I have nothing to say. I want to make a phone call to my lawyer.")
Also, be aware that if so much as touch a police officer when they don't want you to, you could be tasered or arrested and beaten and stuck with a felony charge for "assaulting" a police officer, especially if the cop had any sort of injury that received medical attention.
So, in short, if you are stopped by police and even if you think you didn't do anything wrong, submit to their authority and surrender rather than resisting - even to try to explain yoursel - or you could face resisting arrest charges on top of whatever else they charge you with.