When a hot molecule reaches a cold one, the hot one gets a little colder, and the cold one gets a little hotter.

Saying that one molecule (I'll call it molecule A) is hot compared to another molecule of the same kind (I'll call it molecule B) means that A has more energy than B. If you want a rough picture of this, you can think of A as a rolling ball and B as a stationary ball. If ball A hits ball B, ball B will start rolling a bit, and ball A will slow down.

In the same way, molecule A reaching molecule B results in a transfer of some of its energy, so molecule A gets colder (slows down), while molecule B heats up (starts moving).