when concentration is required, it often means that the spell is fairly complex.

involves maintaining focus over time.

the Sunlight Charm requires that you
continue to concentrate upon the spell after it's been cast. You need to
concentrate upon the target of the beam of light as well as the
intensity of the light, otherwise it will flicker out within moments.

you may also have to concentrate on a
particular thought, feeling or desire while casting the spell. The best
example is the Patronus Charm. To cast this spell, you need to
concentrate upon a very happy, very powerful memory.

Modifiers

Not all spells can be modified.

Sunlight Charm

The typical Wand-Lighting Charm
Allows you to light the tip of your wand, move it as you want and cast
other spells, and then counter the charm with a single simple
counterspell. Sometimes, however, it's useful to have a brighter light
that's focused in a particular direction. Instead of a candle casting
light in every direction, the Sunlight Charm is like a very powerful
flashlight.

Dancing Light Charm

allows you to conjure several smaller
lights. This experimental charm is great practice for concentrating on
multiple things at once. Since each light must take up some measure of
your concentration, there are only so many lights that can be conjured
with the spell. To move the orbs of light around you also need to
concentrate, which further limits the number that you are able to
conjure.