Mmm. Well, the thing about Premiere Elements is that you've used it more than we have.
Anyway, the quickest way to understand keyframes is to forget that word for a minute. Zap it back out of existence in your brain.
So, in creative software, you adjust the properties of things — like what color your text is — until you've made it the way you want it. In static graphics, the story ends there. "I want red text." Photoshop is like "like this?," and you're like "yeah, that's super red, thanks." End of story. But, in graphics where time is a component, sometimes you want the color of the text to change over the course of your shot. Maybe you want it to start red, and end blue. You tell the computer "hey, red at the beginning is fine, but make it blue by the end," and the computer interpolates the necessary color changery to animate the text from red to blue over the course of your shot.
We call those directions that you gave the computer "keyframes." In this example, you set two of 'em: a keyframe at the beginning of the shot, which says "make the text color all the way red," and a keyframe at the end of the shot, which says "make the text color all the way blue," and over the course of the shot, that property (text color) goes from one to the other. At the halfway point, the text would be half-red half-blue, and so on.
It's just a little point of value data on a timeline, so you can change the value over time. You say "at Time A, I want Property B to be Value C", and so it is. If you didn't have another keyframe for that property, it'd just stay Value C forever. (And in that case, you wouldn't even need to "activate" keyframing anyway, because you could just... set it at Value C like you would in Photoshop and move on with your miserable afternoon.)
Anyway, eventually we all die in the end, but you knew that already. Questions? I can do this schpiel on video with a whiteboard if ya want.
Teague Chrystie
I have a tendency to fix your typos.