What is failure?
Going to failure in an exercise means doing a certain exercise until you simply can't continue, which could end up halfway through.
It is not recommended to overuse this technique as it is very taxing on the CNS (Central Nervous System).
Example
Are you doing flat bench press and you've already done 7 repetitions.
You advance to the 8th repetition but you can't lift any more, the repetition is halfway through and someone has to help you place the bar on the rack.
This is going to failure.