If you usually do something, that’s what you do on a normal day. Tonight you might be making a seven-course meal to celebrate your birthday, even though usually you just get take-out pizza.
Usual comes from the word use, and implies "the most common use." A screwdriver usually drives screws into something, or takes them out. Unusually, you have strung your screwdriver on a chain and are wearing it as a necklace. You can also say usually to describe the normal state of things: "On New Year’s Eve, this usually quiet street erupts with noise."