The random number syntax generates an integer value within the given range (inclusive of the minimum and maximum value). It can be used in place of a literal value for most function arguments where a numeric value is expected.

wait(random(1, 5))

Waits for a random amount of time between 1 and 5 seconds.

if random(0, 100) < 25 then
  give_item("ITEM_0001")
else
  give_item("ITEM_0002")
end

Gives either ITEM_0001 or ITEM_0002 to the player, with a 25% chance that the item will be ITEM_0001.