'Explicit' is the opposite of 'implicit'. If something is implicit, it is implied or suggested. By contrast, if something is explicit, it is stated or shown.
So, for example, if you see a couple kissing passionately in a film and then in the next scene you see them asleep in the same bed, what went on between these two scenes is implicit. The film doesn't show us what happened, but we can imagine. A film like this does not have 'explicit content', so it is suitable for underage viewers.
A film which shows the scene in the middle, however, would be classed as 'explicit content', because it leaves nothing to the imagination. In other words, it generally means scenes of sex and nudity. A cinema would not be allowed to admit viewers under a certain age to such a film. On the TV and internet you have to have a warning.