What's "sneak up" mean?

Native speaker’s answer
Rebecca
To "sneak up" means to move very quietly or sneakily, to prevent others from seeing or hearing you. In this case, it's used to express that some people drive down the shoulders (emergency lanes) to sneak up on other cars and get ahead of the traffic. Ex: Wow, don't sneak up on me like that! You scared me. Ex: He snuck up on his friend and tried to scare him by yelling "boo!"