Gravity is the missing piece of the Standard Model puzzle. It is also integral to Einstein's
theory of general relativity. General relativity postulates that time and space are not separate
entities but two parts of a singular manifold: spacetime.
Spacetime is like a fabric or a sheet that stretchs out across our universe. This sheet is not flat
but warped, as objects- like the sun, planets, moons- are placed on it. Their mass causes it to curve, like so:
A more massive object will rest at the center, attracting less massive objects towards it. This is why Earth and other planets in our solar system orbit the sun, or why the moon orbits Earth.
Orbit, in the sense of massive objects pulling less massive objects towards them, sounds a lot like gravity, doesn't it? Well, gravity, in terms of general relativity, is the warping of space time. What's more, the warping of space time gives rise to mind-blowing phenomena, such as time dilation (another Fermilab gem).