https://duoplanet.com/duolingo-streak-freeze/

what is a streak freeze?
A streak freeze on Duolingo allows you to pause (or in this case ‘freeze’) your streak.

This means that if you miss a day — i.e. you don’t login and complete any lessons — then your streak won’t reset to 0.

A streak freeze is basically a safety net that gives your streak that extra bit of protection.

If you miss a day while a streak freeze is active, your streak will remain exactly the same, meaning you won’t lose it, but you won’t gain a day either.

For a streak freeze to work, you need to have one active before missing a day. It’s not something you activate after losing your streak!