What do you mean by SQL Server Parameter sniffing

When we work on a project having huge database interaction we use stored procedure to get good performance. Stored procedure performance was not so good all the time. I mean it was not consistent for example few queries on same stored procedure was so good and some times it was going worst.This condition occur when we have a stored procedure and the amount of data returned is varying wildly depending on what parameters are passed in.Some time you will see
that the performance of stored procedure change dramatically depending on how much data is being returned. Then you could be a victim of SQL Server Parameter sniffing.