Maturity period is the agreed upon time period after which the investment ends. This usually means that you will get your settlement or be able to access or encash the full amount/ asset after this date.

Example:
If you opened your PPF account in 2009 then as per the scheme rules, the account will mature after 15 years and you could withdraw the complete amount without any penalty or special request. The maturity date of PPF account will be 2024 and the 15 year time between 2009-24 is called maturity period.