I'm talking about the "endurance" spec. Because Flash Memory can only be erased a finite number of times[1], SSD manufacturers specify the drive's endurance -- defined as the number of bytes which can be written to the SSD over its lifetime.
The SSDs helpfully keep track of how many bytes you've written and report that in the SMART info. For example, on my Windows dev system, HDD Guardian reports that I've used up 42% of my SSDs endurance (it's a Crucial m4 512GB). So by "usage", I mean the percentage of the endurance which has been burned.
[1] Each block of flash memory must be erased before it can be written. Each time you erase it, it "uses it up" a bit and will be harder to erase next time. So each time, the SSD controller is forced to erase it with just a bit more voltage. As the blocks become harder to erase, it takes more time to erase them and the SSD write bandwidth decreases.
