What does it mean by this, "However, no object is truly stationary, so all motion is relative motion."

the distances between things change (this is motion)

you have to pick your scale, but it is "all relative"

at home, sitting in your chair; you may say you are not moving
you would say that a person walking by is moving
you are using the Earth as your reference frame

driving next to another car on the highway
you are both going along the road
but you are not moving relative to each other

what about the Sun...is it moving...and why

your home reference frame is relative

everything moves, relative to something