Espresso is about ratio and time.
Around 1:2 — 18 g of coffee for 36 g in the cup — in 25–30 s. Fast and sour? Grind finer. Slow and bitter? Grind coarser.
What it is
Espresso is a small, concentrated coffee made by forcing hot water through a tightly packed bed of very finely ground coffee, under pressure from a machine.
Why it matters
Two numbers tell you almost everything: the ratio of coffee in to drink out, and the time it takes. A common starting point is 1:2 — 18 g in, 36 g out — in 25–30 seconds. Change one thing at a time and taste.
In practice
Weigh the dose into the basket, spread it evenly, tamp it level and firmly. Put a scale under the cup, start the shot and stop at the target weight. Fast and sour? Grind finer. Slow and bitter? Grind coarser. Once the time is right, a longer ratio (1:2.5) tastes lighter; a shorter one (1:1.5) richer.