Coffee brewing requires following strict recipes. This includes brewing temperature, coffee grind size, brewing time and the coffee dose.
Coffee dose refers to the amount of ground coffee used for brewing coffee. This, like yield is measured in grams rather than volume to enable baristas to get a precise measurement.
Even though baristas sometimes use volumetric measurement to approximate the amount of grounds that will go into the brew, the recipes are created using weight measurements. Volume approximations are acceptable when the barista uses the same coffee beans. The roast level, the origin, and the coffee variety can determine the density of the coffee bean, hence volume can vary with the same weight.
The coffee dose is the amount of ground coffee that is measured into the portafilter before brewing.
This is not to be confused with the coffee yield, which refers to the amount of extracted espresso after brewing.
The dose is therefore a solid (ground coffee beans) measurement whereas the yield is a liquid (brewed coffee) measurement.
The coffee dose is one of the factors, along with the yield, water volume and grind size that influences the final coffee extraction. The dose weight is typically adjusted marginally if the extraction is very close to being right and the barista feels that the grind size is correct. Adjusting the dose allows for a very minor adjustment whereas adjusting the grind changes the extraction more considerably.