SWR tells you how well the radio is transferring energy into the antenna. While an antenna with a high SWR that transfers energy poorly might not work well, if what does get out goes where you want it to go, it's a better antenna. But remember that a dummy load has a perfect SWR and doesn't radiate at all, so SWR is not the only question.
Typically, a bigger concern is that if the SWR is high and you don't have a tuner, that means your radio is going to overheat because it has to eat some of the energy bounced back from the antenna.
Your goal is to make contacts, meaning the energy gets not only out of the antenna but to someone else who can hear it. There are at least three ways that energy goes wrong:
- High SWR means it doesn't leave the antenna, but bounces back to the radio (and bounces off the radio back to the antenna and back to the radio...and a little escapes at each end on each bounce).
- Low efficiency ironically can improve SWR, but it means that the energy is turned into heat instead of radiating out of the antenna. (Loss resistance vs. radiation resistance.) This is your dummy load.
- Poor radiation pattern -- either the antenna is isotropic, radiating equally poorly in all directions, or the direction it is radiating is different from where you want it to go. For example, up instead of horizontally, or east west when you want north south, etc.
For example, I have a disc cone antenna that has a 1.0 swr from 140mhz - 900+mhz. At 2m, it does pretty well, maybe with a gain of 3. But at 440mhz, the swr is still 1.0, but the gain is maybe 0.7 - 1, because the radiation pattern lifts up as the frequency goes up, sending more into the sky and less to stations on the ground.
If you have a horizontal antenna above the physical ground, if that ground is a poor conductor, then it is a resistor. As the antenna gets closer to the ground, more of the energy is absorbed by the ground and less is radiated, so the SWR goes down but so does the gain. Additionally, if the ground is reflective, then as the antenna gets closer to the ground, more energy goes up into the sky (NVIS?) and less towards the horizon.
Low SWR is good for the radio and good for your feed line (both of which absorb more at high swr), but good for the antenna is resonant -- which may or may not be where SWR is low (as explained in the other answer). And good for transmission depends on radiation pattern in addition to resonance.