The text below has been copied from the Dolmetsch website. Having read it I'm not sure that it'll make it any clearer for you, but as I said above the Josephine Koh book explains it well and there are lots of exercises to do. If it's any consolation I did Grade 7 theory and didn't come across this in the paper I sat or in the past papers I did, so I wouldn't lose any sleep over it.
(Tonicization) "also called 'false modulation' or 'transitory modulation', tonicization is a process that temporarily allows a chord other than the tonic to function as a goal of motion or point of stability, and therefore, function as a temporary tonic. A chord is said to be "tonicized," when it is preceded by its own dominant, dominant seventh, seven chord, or diminished seventh, that is, the dominant determined by the key of the chord. Tonicization is a local event, unlike modulation, which implies establishing a new key centre and continuing in the new key. Any chord in a major key, except VII, can be preceded by its own dominant. Any chord in the natural minor, except II, can be preceded by its own dominant"