Pressure altitude is what the altimeter says with 29.92 inHg dialed in. Density altitude is what the wings, the engine, and the propeller actually feel — and only one of those two numbers cares whether it's a cold morning or a hot afternoon.
A pilot at a 5,000-foot-elevation airport might reasonably expect "high-altitude" performance every day the field is open — thinner air, longer takeoff rolls, reduced climb. But that airport doesn't have one performance number; it has a different one every hour, and the field elevation barely moves the needle compared to the outside air temperature. On a cold winter morning, the air over that runway can be dense enough that the airplane performs close to a sea-level day. On a hot summer afternoon, the same runway, the same altimeter setting, the same physical elevation can behave like an airport thousands of feet higher than it actually sits. The number that predicts which of those two very different days you're getting is density altitude — and it is not the same thing as pressure altitude, even though pilots often use the two loosely as if they were interchangeable.