What does the mass of a body measure?

$A$ man getting down from a running bus falls forward because:

$A$ reference frame attached to the Earth cannot be an inertial frame because

Due to which property of an object does it resist any attempt to change its state of motion?

Inertia is that property of a body by virtue of which the body is

$A$ person is sitting in a travelling train and facing the engine. He tosses up a coin and the coin falls behind him. It can be concluded that the train is