It’s a coin-cell battery. Traditionally it was used to keep the memory that stores the bios-settings and the real-time-clock powered when the PC was turned off.
By now the bios settings are stored ona different kind of memory, so it doesn’t need power when turned off.
But the rtc still needs power when the laptop is off as well as other stuff (for example the circuitry that makes the power-button work)
In the framework it’s also rechargeable, so you can’t just swap it for a cheap one from the store once it runs out.
The battery lasts about a mont in that laptop and gets worse quickly over time when not regularly charged.
I’m not sure if the short runtime is caused by the design-decision of using a rechargeable battery or a big power-draw from it.
For me this is also the first laptop that ever had an issue like that. Even my decade old thinkpad is still on its first CMOS battery.