Lift is the atmospheric process that pushes air upward against its own weight, and it is what turns a moist, unstable environment into an actual storm. Instability describes what a rising parcel would do if it were displaced. Lift is what does the displacing.
How air gets forced upward
Air near the ground almost always sits under some amount of resistance, either warm air aloft capping the column or simple negative buoyancy near the surface. Something has to push a parcel past that resistance before free ascent can begin.
Fronts do this by wedging cooler, denser air under warmer air and forcing the warm air to climb. Drylines do it along the moisture gradient, where hotter and drier air undercuts moister air and lifts the moist side. Terrain does it any time low-level flow runs uphill. Thunderstorm outflow boundaries do it after the fact, acting as small cold fronts that can trigger new storms hours or even a day after they formed.
Above the boundary layer, divergence in the jet stream removes mass aloft and forces compensating ascent underneath. Shortwave troughs, mesoscale convective vortices, and gravity waves all provide similar synoptic and mesoscale lift by rearranging mass in the column.
How forecasters read lift
Surface analyses locate boundaries. Upper air charts locate jet streaks and troughs. Satellite trend loops show cumulus fields tightening along a boundary, which is often the earliest visible sign that lift is beginning to work through the cap.
The strength of the lift matters as much as its presence. A weak boundary in a strongly capped environment often fails to produce anything. A vigorous shortwave lifting a marginally unstable airmass can still produce widespread storms if the moisture is deep enough to survive the ride upward.
Where lift misleads
Lift on a map is not lift at the storm scale. Synoptic ascent is smooth and slow, on the order of centimeters per second, and by itself will not break a cap. What actually initiates storms is usually the sharp, boundary-scale convergence that superimposes on top of the broader ascent.
Lift also does nothing on its own. A strong front moving through dry, stable air produces cloud lines and not much else. Treat lift as one required ingredient, not as a trigger that guarantees a storm.