Of course, there are many things that you could sit in, and there are other things you could use a chair for. There also are infinitely different kinds, colors, and types of chairs. Yet, there is some quality that all chairs must have in order for them to be defined as chairs. Plato would call this quality a Form. If there is a Form by which chairs are chairs, there must be a Form by which other things are other things.
Tables have an overruling Form, as do stools, desks, computers, paper clips, etc.
Anselm recognized this and decided that there must be a Form of existence as well. This is existence itself. Anselm defines God as that being which confers existence upon all. That is, there is something known as goodness, by which other things partake and are therefore found good, though in a lesser degree. There is a Form of goodness, also known as God.