Now try to make two scripts for your hero that start with the same "hat" (that's how the control commands are called). For example:
Can you tell what will say first?
These scripts are said to run concurrently, that is, it depends on the Scratch platform which will be done first and we cannot safely define. We cannot determine when a player starts playing our game which of the two will happen first.
In professional programs, concurrent isuues can be a reason why a program "crashes" sometimes but not always.