If we specify the READPAST hint in the SQL queries, the database engine ignores the rows locked by other transactions while reading data. Suppose you have a transaction that blocked a few rows in a table for updating the information in those rows. Now, if another user starts a transaction and specifies the READPAST query hint, the query engine ignores these rows and returns the remaining rows satisfying the data requirement of the query. It might return incorrect data as well.
There are some very limited uses for this hint, though they are out there.