We mentioned how activation energy is like an energy hump a reaction has to go over. The first plot above shows an Sn2 reaction you might find in organic chemistry: OH- attacks a bromoethane molecule, kicking out the bromine and making ethanol. Here’s the important part: carbon (C) likes being bonded to 4 things, but in the transition state at the top of the hump, it’s kind of bonded to 5 things. It doesn’t like this, but has to go through it for the reaction to happen.

Notice I said the carbon doesn’t “like” something. In science-talk that means it “has high energy.” You can think of it like this: if your room is filthy, you might think “I’d be happier if I cleaned my room.” But to get to that happier state, you have to clean, which requires effort and generally doesn’t make you happy. If you just can’t seem to bring yourself to clean, it’s because the activation energy is high. You just can’t get over that hump.