Most chemical reactions do not happen in one single giant step. Instead, they occur through a series of smaller events called elementary steps. Each elementary step involves a specific number of molecules crashing into each other.
Chemists call this specific collision count the molecularity of the reaction step. Sometimes a step creates a temporary molecule called a reaction intermediate. This intermediate gets completely consumed by the time the next step finishes.
Because they disappear, intermediates never show up in the final balanced chemical equation. When you add all the individual elementary steps together, they equal the overall reaction. In any mechanism, one specific step will always be the slowest part.
Chemists call this bottleneck the rate-determining step of the entire reaction. This single slow step completely controls the speed of the entire chemical process. Knowing the mechanism helps chemists understand why reactions happen at certain speeds.
It also explains how changing the temperature or adding a catalyst changes the speed. Students often think they can guess the rate law directly from the overall equation. You can only write the rate law if you know the slowest elementary step.