For example for text search on a set of documents recall is the number of correct results divided by the number of results that should have been returned
In binary classification, recall is called sensitivity. So it can be looked at as the probability that a relevant document is retrieved by the query.
It is trivial to achieve recall of 100% by returning all documents in response to any query. Therefore, recall alone is not enough but one needs to measure the number of non-relevant documents also, for example by computing the precision.