Newton's first law says that an object stays at rest, or keeps moving at a constant velocity, unless a resultant force acts on it. Constant velocity, also called uniform velocity, means the same speed in the same direction. The resultant force is the single force that has the same effect as all the forces on the object together.
When the forces on an object are balanced, the resultant force is zero. A stationary object stays stationary. A moving object keeps moving in a straight line at the same speed. This means no force is needed to keep an object moving. A car moving at a steady speed on a straight road still has forces on it, but the driving force is equal to the resistive forces, so they balance.
When the forces are unbalanced, there is a resultant force and the velocity changes. The object can speed up, slow down or change direction. Velocity includes direction, so an object moving at a steady speed in a circle has a changing velocity, and there must be a resultant force on it.
