consider agro as the hate of monster toward you.
if u play in a party..
as tanker..u will want all the agro.. so monster will attack you.
and as other..you may not want to agro more than tanker.
so u cant just go do crazy damage.. must man man..let tanker build up agro first..

nice eh..

not always true..
give massive damage but do not over agro. (as the topic intended to tell lol)..
as simple as keep the tanker agro always in highest. (slowly damage the mob..more dots..more dots..more dots..)
but this only apply to certain(boss) fight..
keep on playing.. you will understand later ...
nothing teach better than hands on.