Newton's First Law
Newton's first law states that: "In the absence of external forces, an object at rest remains at rest and an object in motion continues in motion with a constant velocity (that is, with a constant speed in a straight line)". What Newton is basically saying with this, is that an object does not accelerate when there is no force on that object. From this it is concluded that an object that is isolated from its environment will either remain at rest, or remain at a constant velocity.
An object may have a tendency to resist a force acting on it. This tendency is known as the inertia of an object. Newton refers to each object as being part of an inertial frame. These frames may be part of a reference point that is not accelerating itself.
This idea of thinking was extremely radical for the time. This is at a time when there was still so much to be learned about the universe and how things worked. For a claim like this to be made, it was really extraordinary.
Here's a link to a really cool diagram that explains all the different concepts behind Newton's first law, I would highly recommend checking it out! http://hypertextbook.com/physics/mechanics/newton-first/concept-map-1.pdf