Out of sample in Forex is a free lecture from the course Forex money management strategy: Top 10 NZDUSD EAs. The complete course is available here:
https://eaforexacademy.com/courses/algorithmic-trading/forex-money-management-strategy-top-10-nzdusd-eas/

The Out of Sample allows the traders to test Expert Advisors on unknown Historical data. This way the over-optimized strategies are being removed.

For more algorithmic trading courses:
https://eaforexacademy.com/courses/algorithmic-trading/

If you have any questions:
https://eaforexacademy.com/forums/forum/general/