What is Minecraft Realms?
Minecraft Realms is an official service from Mojang (the creators of Minecraft), which gives you:

🎮 Your own dedicated Minecraft server - runs 24/7, even when you're offline
👥 Play with friends online (up to 10 players on Bedrock Edition)
🌍 Persistent world - your builds and progress are saved between sessions
🔒 Private server - you decide who to let in
🛠️ Easy management through the official interface (no port settings, hosting, etc.)

What exactly does the reward provide?
1 month free Realms subscription, meaning:

Redeem the code/reward on your Microsoft/Xbox account
Get access to one Realms server for 30 days
After one month, the subscription simply ends (it does not renew automatically)