Plain English Meaning
How far the rudder can swing left or right when you push the pedals all the way. 'Full rudder travel' means you've pressed the pedal as far as it will go.
Full Definition
The range of movement, measured in degrees of deflection, that the rudder pedals can produce in the rudder surface from neutral to full deflection in either direction. In a slip, the pilot may use full available rudder travel to maintain a desired heading or sideslip angle.
Context Anchor
Seen in discussions of intentional slips, crosswind landings, and flight- checks, especially when large rudder input may be needed.