Reference Point
A reference point is a fixed object or location used to decide whether something else is moving and to measure its position.
Definition
A reference point is something that stays put so you can tell whether other things are moving, like a landmark you compare everything else to. In physics it is an object or location assumed to be stationary, from which position and motion are measured, with position stated as a distance and direction from that point. Changing the reference point can change whether an object appears to move at all. At the most formal level, a reference point is the origin of a coordinate system, and together with a set of axes it defines a reference frame. Position vectors, velocities, and displacements all depend on the frame chosen, and frames moving at constant velocity relative to one another are called inertial, with Newton's laws holding in every one of them.
Example
When you look out a car window, a tree by the road is a good reference point: the tree seems to slide backward, which tells you the car is moving forward. Using the school flagpole as a reference point, a cyclist who is $50\ \text{m}$ east of it at noon and $350\ \text{m}$ east one minute later moved $300\ \text{m}$ east in $60\ \text{s}$, or $5\ \text{m/s}$. Now picture a passenger walking toward the front of a train at $1\ \text{m/s}$ while the train moves at $30\ \text{m/s}$. Relative to the train, with the origin fixed to a seat, her velocity is $1\ \text{m/s}$; relative to the ground, with the origin fixed to the platform, it is $31\ \text{m/s}$. Both are correct in their own frames.
Key Insight
Without a reference point you cannot say whether anything is moving; motion is always a matter of "compared to what?" The most convenient reference points are things that do not move relative to the ground, such as buildings, signs, or trees, but picking a moving reference point like another car gives a different and equally valid description. Choosing a clever reference frame can even turn a hard problem into an easy one. Collision problems are often simplest in the center-of-mass frame, where the total momentum is zero and the algebra collapses.