What is the rate-determining step?

The rate-determining step is simply the slowest step in a reaction mechanism of a multi-step reaction. 

Most reactions do not happen in one simple step. You may be familiar with overall reactions however these are usually carried out over multiple steps. The rate-determining step is the one which takes the longest.

One way you can think of it is to imagine two people washing up. One person is washing the dishes and the other is drying them. Even if the person drying them is very fast, the whole process will be slow if the person doing the washing is slow.