To intensify is to increase the intensity, force, or power of something. If you snap a selfie and the colors aren't intense enough, you may be able to intensify them with a photo editor.
You can see the word "intense" in intensify, which clues you in to the meaning of the verb. When you make something more intense, you intensify it — make it more of whatever it is. Believe it or not, a pinch of salt can actually intensify the sweetness of the sugar in a cake. It makes it even sweeter. Storm chasers never know whether a tornado will continue to intensify or begin to abate as they approach — and the difference could mean life or death for them.