Gravity wells - 

A Gravity well or gravitational well is defined as “a conceptual model of the gravitational field surrounding a body in space.”

The more massive the body, the deeper and more extensive the gravity well associated with it. The Sun is very massive, relative to other bodies in the Solar System, so its gravity well appears “deep” and far-reaching.

(picture a very heavy object sinking deep into a bed mattress; the more mass the object has the deeper it sinks in and creates a deeper sinkhole; a deeper sink hole will pull in any nearby objects towards the centre object with greater influence. Objects of  mass bend the fabric of spacetime this way also as the theory of general relativity explains) 

a video example