Molecules must collide with enough force to start a chemical reaction. When they crash together, they briefly form a highly unstable structure. Chemists call this temporary structure the transition state or the activated complex.
This complex sits at the very peak of the reaction energy hill. The molecules can either fall back into reactants or roll forward into products. The speed of the whole reaction depends on how many activated complexes form.
It also depends on how fast those complexes break apart into the final products. This theory gives a much better picture of reactions than older models. Older theories only looked at how often molecules bumped into each other.
Transition state theory looks at the actual shapes and energies of the colliding molecules. A common student mistake is thinking you can trap or store a transition state. You cannot capture or put an activated complex in a glass bottle. It only exists for a tiny fraction of a second during the actual crash.