The highest or uppermost part of something is its top, like the top rung of a ladder or the top bunk of a bunkbed.
A top is also a lid, like the top of a soda bottle, an upper rank, like the top position at a law firm, and a child's spinning toy. If you yell at the top of your voice, you use the loudest possible volume. As a verb, top can mean "to beat in a competition," like when you top all of the other contestants in the spelling bee, or "to provide with a topping," like when you top your pie with whipped cream. Your band director might command, "From the top!" meaning "Play it again, from the beginning!"