Understanding molecule infrared absorption

My chemistry course requires us to know which molecules absorb infrared, and under which vibrational modes that they do. My professor provides us the following rule, which is very confusing for me and I still don't know how to start.

Rules for IR absorption: (1) The frequency of the IR radiation matches the frequency of the vibrational mode. AND (2) There is a net change in the symmetry of electrical charges during the vibrational mode.

So, say we have a molecule such as CF2Cl2. This absorbs infrared under all vibrational modes: symmetric stretching, asymmetric stretching, and bending. How do you get to that answer?

Thank you very much.

Molecules such as benzene vibrate in many ($3N-6$) normal modes, some such as asymmetric stretches produce transient dipoles and have IR transitions. Others do not and so have no transitions. What determines the are called selection rules.
(The molecule you mention belongs to the $\ce{C_{2v}}$ point group, see molecule-viewer.com for a 3D model with symmetry elements.)