Page yield is the estimated number of pages a cartridge will print before it needs to be replaced. It is based on the amount of ink or toner within the cartridge and how much the printer uses. There are other factors that influence page yield like the quality of the paper used. It's not an exact number, but it's a close estimation.
The most common description of cartridge capacity is "number of printed pages at 5% coverage". This means it estimates the number of pages a cartridge would print at 5% page coverage which is the equivalent of 6 lines of text.
Ink and toner cartridges come in standard sized cartridges and high yield cartridges. High yield toner and high yield ink commonly have "XL" marked with their cartridge model number.
If you have heavy printing needs, check if your printer model supports high yield cartridges.
In page yield comparison between high yield toner and high yield ink, the former has higher page yield.