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If you don't terminate, the signal will be reflected from the monitor input back towards the source. If the source has a good, accurate 75 ohm impedance it will absorb that reflection and no harm is done. Otherwise part or all of the signal will be reflected back towards the monitor. Depending on cable length this can cause frequency response errors or possibly ghost images.
In practice, it's often not a problem if a monitor is left unterminated. The frequency response errors and ghost images will often be negligible.Most monitors will tolerate excess video amplitude. There's no question of stressing or harming a monitor with 2Vp-p of video. But just because you can often get away without terminating doesn't make it right.
The fun starts when somebody puts a 50 ohm termination in the box of 75 ohm ones. This can be malicious, by accident or because you're in an environment where both 50 and 75 ohm equipment is being used.
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