Moment of inertia, abbreviated as I, (English area unit \(in^4\), Metric area unit \(mm^4\)), (English mass unit \(lbm \; ft \;sec^2\), Metric mass unit \(kg \;m^2\)) also called rotational inertia, measures the resists or change an object has to rotational acceleration about an axis. The larger the moment of inretia the more difficult to try to get an object moving and the smaller the moment of inretia the easier of relatively easier to get an object moving.
Inertia is the tendency of an object in motion to remain in motion or an object at rest to remain at rest unless acted upon by a force. The larger the mass of an object the more resistance to change in motion than objects of a lesser mass. It is the tendency of objects to keep moving in a straight line at a constant velocity and direction forever unless acted upon by gravity or another force.
First moment of area, also called first moment of inertia, is the resistance of an object to bend around a certain axis. This is more non engineering related.
Second moment of area, abbreviated as I, also called second area moment or area moment of inertia, is the resistance of an object to bend around a certain axis of a cross-section area. This is used in engineering of beams and supports.