what that means is that it will check to see if either the specified conditions at the top (anybody,present for example) or the boolean variable skip is true. if either is true, the trigger will run
But this is the answer to you're question:
ANY condition can be any form of code you want, as long as it returns ONLY a boolean value
for example
trigger:
east, not present
condition: this and (unit1 distance unit2 <= 50) and (varible1 - variable2 == 7.5) and somebooleanvariable
on activation: unit2 setdammage 1
what that does is checks for east not present and unit1 to be 50 or less from unit2 and one variable (a number) - another variable (also a number) = 7.5 and that a boolean varible (true/false) is true. if any of those are false, the trigger won't run. if you changed all of the AND's to OR, it would only need one of those to be true. i can give you more examples if you want, but its pretty straightforward