Inertia in physics means the resistance of a body to any alteration in its state of motion. This can be either the resistance of a body at rest to being set in motion or the resistance of a body in motion to any change of speed or of direction.
In physics this is neither good nor bad, it just is. But if you want to use this idea for Life, for emotions, feelings, and human behaviour, then you can think of someone with a lot of "emotional inertia" as someone who is set in their ways and does not want to change. Inertia can be good if you want to be steady like a rock and never change. Or inertia can be bad if you are headed on a disastrous path to a tragic ending but you refuse to change anything about your behaviour or what you are doing.