Mili amp hours. 2600 mah is 2.6 amps.
It is the capacity of the battery.
The capacity determines the
batteries run time.
If your device were to draw 1amp, you would in theory get 2.6 hours of constant runtime.
But
vaping doesn't really draw constant power, like say a flashlight does.
Depending on how much you vape, it could last you a day, maybe 2, or if you are a chain vaper, it may only get you through a workday.
It really all boils down to how much you suck on the thing.