What does an object is at rest mean?

An object is said to be "at rest" when it is not in motion relative to a specified frame of reference. This means that the object's position does not change over time compared to the surroundings or the observer. In practical terms, if you were to observe an object and see that it does not move with respect to nearby objects or a coordinate system, you would conclude it is at rest.

In physics, particularly in mechanics, an object at rest has a velocity of zero; it stays in the same position unless acted upon by an external force, in accordance with Newton's first law of motion, which states that an object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force.