Pluto's orbit is more eccentric than any of the major planets. What does that mean?

Eccentricity -

Eccentricity is a measure, that how an orbit deviates from a circular shape.

A perfect circular orbit, has an eccentricity value equal to zero.

And more the value of eccentricity, more elliptical is the path of the orbit.

Planets - Venus, Earth, Neptune in the Solar System are the most elliptical, hence, Have the least eccentricity value.

And,

Pluto, in the solar system have more eccentricity value.

The value of eccentricity of Pluto is 0.248.