Think of the SHA3-256 as a big machine in a toy factory that makes special secret messages. This machine has a big room where it does all of its secret work. It also has a small window where it can take in new toys (input) and send out finished secret messages (output).
The size of this small window is what we call the “bitrate”. For the SHA3-256 machine, this window can hold 1088 toy blocks at a time.
So, the bitrate (or block size) of SHA3-256 is like the size of the window on our secret message machine, and it can hold 1088 toy blocks.