A regular mortgage loan almost always has a standard duration of 30 years. This 30 years duration is divided into sub periods (the duration that your interest rate is fixated);
The longer the interest fixed period you choose, the higher the interest rate level. And the longer the security you have that your monthly installments will not increase.
If you choose an initial period of 10 years you know that the interest rate is fixated and that only after 10 years you will be presented a new interest fixed rate proposal from your bank – against the rate of THAT TIME, so the rate could very well deviate from your current one, leading to a higher (or lower) monthly cost level for your next interest fixed period.