The meg- prefix usually denotes that an act is finalised. If you're familiar with the principle of aspects, it'll help a bit. Meg- puts a verb basically in the finshed aspect, meaning it puts focus on that one kiss instead of referring to kisses I give you in general. It's a bit complicated and I don't understand it fully yet as well (I'm not a native), but I can give you an example to give you an idea of what meg- does:
ért - to understand; megért - to comprehend, to grasp a concept.

So megcsókollak pretty much means that I'm going to full-on kiss you and not just because that's what I usually do. The English translation, though, leaves some room for interpretation, and just csókollak is a valid answer as well.