An easy way to remember this (should you ever need to!):
Cousins (�unremoved�) are of the same generation. First cousins share common grandparents; second cousins share common great-grandparents; third cousins common great-great-grandparents, etc. So this puts the successive cousins wider apart but at the same generation level in a family tree.
As naomicorlett suggests, �removal� refers to differing generations. So your �second cousin once removed� is your second cousin�s child.