What does WHERE (wo.ISPARENT='1') mean in a query?

What does WHERE (wo.ISPARENT='1') mean in a query?

I've seen this used a lot in queries provided to users:
WHERE (wo.ISPARENT='1')
It seems to eliminate deleted records from a query.

Is that a proper use for that condition?

Are there other things that field is used for?

Thanks!
Joanna