To do is to act, perform, or undertake. When you do chores, you engage in the activity of cleaning the kitchen or taking out the trash.
Do is one of the most frequently used verbs in English. You do many things in a typical day: you might do the dishes, do your job, and later on, do your taxes. If you meet an old-fashioned gentleman, he might say, "How do you do?" essentially meaning, "Hello." After a difficult test, your friend could ask, "How did you do?" You can also do as you please, or just do nothing all day long.