Event Blocks in Scratch
Events blocks are color-coded burnt orange 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.
There are currently 8 Events blocks: 6 Hat blocks and 2 Stack blocks. It is the smallest category of blocks.
The above information is referenced from Scratch Wiki.