initializes the location of the command with a provided value
For example
(x3001) ~~~~~~~~ LABEL1 .fill 30
(x3002) ~~~~~~~~ LABEL2 .fill 31
x3001 in memory is filled with the value 30, x3002 is filled with 31. We put labels for later access if needed, and .fill lines will usually look like this, but the use of labels isn’t strictly neccessary.