"At" is used for a specific place, time ."He works at the store" or "The party starts at 5pm" It can also be used to indicate activity . "He is good at skiing" or "She jumped at the door"

For most verbs involving motion, "at" is a faster or more violent alternative for "to". The five-year-old girl ran *to* her mother and gave her a hug.--> The soldier ran *at* his enemy and tried to kill him. I drove *to* my friend's house and picked him up in my car. --> Look out! There's a car driving right *at* you. I threw the ball *to* my teammate, and he scored a goal. --> I threw the ball *at* a guy on the other team, and injured him badly. Some other ways that "at" can be used violently: I told a joke, and my friends laughed *with* me. --> I fell down. Some jerks saw it, and laughed *at* me. The teacher talked *to* his students, making sure to explain everything carefully. --> Why should I go to school? I don't want to spend all day sitting in a room while some guy talks *at* me about boring nonsense.