A block is just a section of the script. A script usually has lots of blocks to go with on the jump.
An example would be:
CODE
enemyresources_jump (word) (word) (block)
If enemy has at least %1(word) minerals and %2(word) gas then jump in %3(block).
:block1
enemyresources_jump 5000 5000 block2
goto block3
:block2
build 3 scv
:block3
build 1 scv
Basically if the enemy of the computer has at least 5000 minerals and gas the computer will build 3 scvs. If the enemy of the computer doesn't meet the requirement it will build 1 scv instead.
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That sounds an awful lot like ZZT-OOP.