Making coffee is extracting soluble material from the ground coffee. The more you extract the better it is the cup of coffee, to a point, too much extraction is bad:
(Credit to James Hoffman - Understanding Espresso Video).
Extraction is expressed trough a percentage, is the percentage of coffee grounds you were able to dissolve in the water.
Measuring the Extraction
Until a while ago, you had to carefully extract the puck from the basket, drying it an oven, a weighting it against the weight of coffee you used to make the puck to know how much coffee was dissolved in the cup.
Today we use Refractometers.
However, the more you want to extract, the more work you have to put in. Extracting more, is more difficult.
Quality of equipment is essential to extract properly. A better Coffee Grinder grinds more uniformely and therefore makes it easier to extract larger doses of coffee. SImilarly, better water and temperature dispersion trough the puck makes it easier to extract coffee and this can be achieved with good quality machines.
Another important factor is the coffee roasting. Generally speaking the darker the coffee is the easier it is to extract.
Also, as a rule of thumb, higher elevation grown coffee it’s harder to extract than low grown coffee, but it’s not an absolute.