Normally it means that your printer and/or its cartridge's logic circuitry, estimates that your ink is getting low, often based simply on the number of pages you've printed. In other words it may not be based on any ability the printer has to measure the actual amount of ink left.
Who knows, this may change in the future. However, in most cases you should be able to continue using a cartridge until the print quality simply becomes unacceptable.
Where the 'Ink Low' message is useful, is letting you know that you should have a back-up cartridge close to hand. In fact, we always recommend that you have a spare so that you're not forced into making a panic purchase, which by their nature can be expensive - as well as stressful!
In all cases we recommend that you read the manufacturers' documentation in relation to your ink product, and understand exactly what an 'Ink Low' message means for the operation of your printer. And do remember to read between the lines. They will be only too pleased if you were to replace this profit-generating consumable prematurely.