When climbing a route that is strenous the climber may "run out of steam" before the end and fall off. Having to place protection whilst climbing greatly contributes to the fatigue experienced.

When in doubt run it out indicates that if you think you might fall off then don't bother placing gear, just try to get to the top. Not normally a good idea, but if the gear is going to be crappy, then perhaps this is an option to be considered.

This is a favourite expression of Ben Moon's but he is a climbing god.