Newton's first law, the law of inertia states:
Objects at rest stay at rest and objects in motion stay in motion, unless acted upon by an outside force.
The motion stated in this law is specifically straight-line. If the motion is not straight, at least one force is acting upon the object.
Inertia is the key word for Newton's First Law. We use it in everyday speech to describe an unwillingness to change or adapt. In physics it means something slightly different. Inertia means that an object can only change it's state of motion with the help of an outside force.
The "outside" force is relative to the scope of the system being observed. For instance, you might be sitting still while reading this, but you are connected to the Earth which is hurtling around the Sun at alarming speed, and you are part of the solar system which is hurtling around the galaxy, which is in turn hurtling through the local galactic cluster (currently the Milky Way is on a collision course with the Andromeda galaxy, but the two won't start to collide until after the Sun dies). All this motion goes unnoticed because it is well outside our natural scope of observation.
It is generally unintuitive to think of an object constantly moving ins a straight line forever. In our everyday experience, this never seems to happen. On Earth, it is difficult to isolate an object in a system with no outside forces. If you move an object over a surface, that surface, no matter how polished an slippery, will exert a force of friction on the object, slowing it and changing its motion. If an object is moved in air, the air molecules will bump against it, causing air resistance. A vehicle moving across a road must constantly exert a force from the engine to counteract friction and maintain a constant speed.
When we look at objects in motion in the vacuum of space, unadulterated by everyday forces, we begin to see the effects of the First Law clearly. Objects launched from the Earth with enough initial force to escape the Earth's gravity will travel in a straight line forever unless they pass near enough to a large object such as another planet that the force of gravity from that body will act on the object.
