In celestial mechanics, secular refers to the long-term motion of a system, and resonance is periods or frequencies being a simple numerical ratio of small integers.
Typically, the synchronized precessions in secular resonances are between the rates of change of the argument of the periapses or the rates of change of the longitude of the ascending nodes of two system bodies.
Over relatively short time periods (a million years or so), a secular resonance will change the eccentricity and the inclination of the small body.
Asteroids that approach Saturn have their eccentricity slowly increased until they become Mars-crossers, when they are usually ejected from the asteroid belt by a close encounter with Mars.
The resonance forms the inner and "side" boundaries of the asteroid belt around 2 AU and at inclinations of about 20°.