It means is that there is a location on your map called 1PGO (probably means 'First Player Start', so check near the beginning of the map for it). When the conditions for that trigger are met, a marine owned by Player 8 will be created at that location.
However, since the trigger is for Player 1, that won't happen if no one was in the Player 1 slot, which is why critical triggers should always be put on computer or neutral players whenever possible.
Hopefully that helps, if you need any more assistance, just ask.