A realisation - at leat the way I think you mean it is really a deep or profound experience of the truth (or depth) of something.
So I could be contemplating (thinking deeply over) the question 'what do I fear' and then suddenly I will have a deep experience of a fear of mine and in this way I will gain powerful insight into this question. Realisations are usually accompanied by a tremendous release of energy, that could have you bouncing around for days (depending on the depth of the realisation), but after that could leave you a little depressed and vague and the realisation begins to dissolve and fade.
Weeks or months after even the most profound realisation, it may just seems like some vague dream and will eventually disappear altogether if you haven't worked with it, embodied it, digested it.
So we all have 'little' realisations all the time, about whatever we may be contemplating or thinking about. Realisations also come in degrees, so you may have a succession of realisations about exactly the same thing, but each one will take you a little deeper (be a little more profound).
Big and profound realisations are things like, the realisation of 'absolute freedom', or 'emptiness' or 'cosmic love' or 'cosmic consciouness' ... etc.
The most profound of all realisations, the deepest and most difficult to come by, is the realisation of causal, non-duality, this is often called 'Enlightenment' and if realised, completely and forever changes your relationship to life.
Hope this helps