meaning when the amount of units alive in yourgrp is 0, the trigger goes off.
Isn't that an explanation? ;D
If you wonder wth the condition is, that one is a 'condition count' command. You only get worse explanations from the comref as you can see:
Counts elements in the array for which given condition is true. It is calculated as follows: 1) Set Count to 0. 2) For each element of array assign element as _x and evaluate condition expression. If true, increease count.
The groupnaming is not obligatory but it's an easier way than naming a bunch of people. Alternatively you could just name the leader as grp1leader, then you could use the condition:
{alive _x} count (units group grp1leader) == 0)
Need more?