A magic circle can be as simple as a sphere of energy created in your mind that surrounds you and the space you are working in. It is used as a boundary to keep inside the energy you want, and keep out the energy you don't want. Some people draw one on the floor or set up stones or candles so that they can see it (visual aid). Some people draw symbols or words around it, believing this adds to its strength by using the energy these symbols or words bring forth.
Not everyone uses circles. It is viewed as an option, and is sometimes seen as important only in very serious rituals, but again, it depends on the individual and their own ways. Some use them, some use them occasionally, and others don't use them at all.
Most people who use them, spiritually clean the area they are planning to do the spell or ritual in. This is known as cleansing or "creating sacred space".
I'm trying to explain it as simply as possible because I know english is not your first language.