A sandbag is a big bag of sand. If a hurricane is coming, chuck a sandbag or two by the beach house to keep the water out. Also, to sandbag someone is to trick or bully them into doing something.
Sand is inexpensive and heavy, which is why sandbags are used for jobs like protecting vehicles and buildings during bombings or storms, or balancing the total weight on a boat. When you use sandbag as a verb, it either means to protect with sandbags or to deceive or coerce someone to get something you want. When you pretend that you're terrible at basketball just so you can beat your cousin in one-on-one, you sandbag her.