``Pure thought alone cannot reveal the nature of the universe any more than it can manufacture gold.''
Science is based upon observations. We can observe space and time by measuring them.
Any spatial position can be characterized by three numbers - coordinates. They are usually denoted by letters x, y, and z. Time is represented by the letter t.
Thus, any point in space at every instant in time can be fully described by four numbers: (x,y,z,t).
Different observers may have different sets of coordinates (x,y,z,t). A set of coordinates specific to a particular observer is called a frame of reference.
Not all frames of reference are equal. There is a special subset of all possible frames of reference called inertial reference frames. They are associated with observers that move freely, with no external force acting on them.
In the non-inertial frame of reference there appear fictitious forces such as centrifugal and Coriolis forces. These forces are called inertial forces.
These forces are fictitious in a sense that there is no physical interaction responsible for these forces. However, a person in a non-inertial frame of reference will feel them quite real!