The pool sets its members an easier target than the current Bitcoin network target. Every time a pool member finds a block with a hash less than the pool target they submit the block to the pool controller and thereby earn a “share”. The pool controller checks that the submitted block has a hash less than the pool target.
The process for mining is independent of target. So occasionally, one of the submitted blocks is not only less than the easy pool target, it is also less than the harder Bitcoin network target, so the pool controller can submit that block to the Bitcoin network, receive the block reward and pay out pool members based on shares won by each pool member recently.
