Details from Tether are very limited on this, but this is possibly something like a portable power station type of device (e.g., along the lines of an Ecoflow, Bluetti, etc.) whose batteries can be swapped out. For providing powering in the home (interior LED lighting, TV, laptop, phone charging, etc) where their utility power is unavailable or unaffordable.

A power station that itself does not need to be so portable, and does not need to charge interior batteries is something that can be built quite a bit cheaper than the current portable systems reliant on solar for power, or an A/C power source.

And with swappable batteries, capacity is essentially unlimited since once could possibly have multiple batteries so that there's always a fully charged battery at the ready.

Solar installation on the house + battery storage might be preferable (versus a battery subscription), but many homes are not suitable for solar installation (shade, or rented and no roof acccess, no place for the batteries, etc.) or the cost is prohibitive. This battery (rental) subscription lowers the cost to access the sun.

And this is apparently a bridge ... until Tether becomes the electric utility too:

Though, my guess as to what this really is could be completely off the mark. We'll see.