"Atypical parameters" refers to the fact that SQL Server may cache a plan based on the actual parameter values passed on the first call of a procedure. If those specific parameter values are not typical (of all possible parameter values) the cached plan may not be a good one for other parameter values. Something like that, anyway 🙂
Statistics Used By The Optimizer
Compilation and Re-compilation
Paul White
SQLPerformance.com
SQLkiwi blog
@SQL_Kiwi