Intensifiers are closed class adverbs that modify adjectives. In English, intensifiers include "really", "slightly", "too" and "very" and possible "not at all", as in really big, too big, slightly big, and very big.
Intensifiers in Gaelic include roL (too) and glèL (very). (The L here indicates that the word triggers Lenition on the following word.) So, given the adjective mòr (big), if we want to say very big, we say glè mhòr, too big is ro mhòr.