后退 means 'to step back' or 'to retreat' in a general sense — you can use it for people, vehicles, or situations. It is broader than 撤退 (chè tuì), which implies a planned military or organized withdrawal, and differs from 倒退 (dào tuì), which emphasizes falling back or regressing. For reversing a vehicle, 倒车 (dǎo chē) is the specific term.