Atomic radius is a measure of the size of an atom. There are other measures, like nuclear radius, but atomic radius is the most common. Atomic radius follows periodic trends, like other atomic properties.
When you go across a period to the right, atomic radius DECREASES. This happens because the nuclear charge becomes greater, and there aren't any energy levels being added, so all the electrons are being attracted closer in. However, the radius jumps in group 18, because the elements in that group are chemically stable.
When you go down a group, atomic radius increases. This happens because more energy levels are being added.