Espresso is just coffee brewed under pressure — but "just" is doing a lot of work in that sentence. A pump pushes water at roughly 9 bars through a tightly packed puck of finely ground coffee, extracting oils and solubles fast enough to produce the thick, syrupy shot with a layer of crema on top. Get the grind, dose, or pressure wrong, and you get something sour, bitter, or watery instead.
What separates a good budget machine from a bad one isn't really pressure — nearly everything on the market claims 15 or even 20 bars, well above what's needed. It's temperature stability, how consistently the pump holds pressure through the shot, and whether the steam wand can actually texture milk instead of just heating it. That's where the real gap between a $150 machine and a $450 one shows up.
