Here's a breakdown:

* Every object in a state of uniform motion tends to remain that: This means that an object at rest will stay at rest, and an object moving at a constant speed in a straight line will continue to do so.

* unless an external force applied it: This refers to the fact that an object's state of motion can only be changed by the application of a force.

In other words:

Objects naturally resist changes in their motion. They tend to stay put or keep moving in a straight line at a constant speed unless pushed or pulled by something.