Yarmulkes (or Kippot) are head-coverings, also known as beanies. One who wears a Yarmulke is in effect saying that reality does not begin with me—there is something above, a Higher Power. Hence its name, “yarmulke,” which is a contraction of “yarei malka,” which means awe of the King. It is a symbol of humility and submission to the Divine.
Technically, you can use anything to cover your head—baseball cap, pillowcase, etc. Even those who don’t wear a yarmulke all the time, will generally wear it when praying or studying Torah.
It is also known as “Kippah”, which means “dome”.