Plain English Meaning
When the pilot pushes a rudder pedal, the rudder swings to one side and the air pushes back against it sideways. That sideways push is what swings the tail one way and the nose the other.
Full Definition
The horizontal component of aerodynamic force produced by the rudder when it is deflected, acting sideways on the vertical stabilizer to yaw the aircraft's nose left or right about its vertical axis.
Context Anchor
Seen when learning how the rudder controls the airplane’s left-right nose movement.
Derivation
"Lift" in doesn't only mean upward — it means the force produced to the a -shaped . The rudder is essentially a small vertical wing, so the force it produces acts sideways rather than upward. Calling it "sideward lift" reminds the it's the same physical principle as , just oriented horizontally.