Why is Newton's first law called inertia?
Aristotle said, "When an object is moving, something external is required to keep it in motion"
This Aristotle's fallacy was believed for years, when finally Galileo extrapolated simple ideas of motion of bodies falling on inclined planes and arrived at the law of inertia which demolished Aristotlean mechanics.
Newton's first law is the same law rephrased thus, "Everybody continues to be in its state of rest or of uniform motion along a straight line unless compelled bu some other body to act otherwise."
Simply, it is stated that "If force on a body is zero, it's acceleration is zero."
Inertia is that property of a body due to which it resists any change in its state of rest or of uniform motion. This law directly lights up the concept of inertia.