Events is one of the ten categories of default Scratch blocks. They are color-coded light yellow and are used to sense events, which trigger scripts to run. Event blocks are essential for every project: without the hat blocks from this category, a project would not be able to begin except by manually running scripts.
Scripts that wear this block will activate once the Green Flag has been clicked — these scripts can activate other scripts and enable the entire program. Without this block, the only way a project could run would be that it would sense the pressing of a key or clicking a sprite; the project would only last until all scripts depending on the starting scripts have ended.