For some reason, ham radio operators cling to an archaic way of measuring whether things are A-OK with an antenna system, called Voltage Standing Wave Ratio, or SWR for short. Most hams treat SWR as dimensionless units, and have no understanding of what's going on under the hood, because the match is painful and it's not well explained.

Faraday was the first to notice electrical standing waves. Before him, standing waves in natural physical phenomena were noticed by early scientists as far back as Archimedes.

Technology moves on though, and today, professionals express reflected power as a result of impedance mismatch (which is how Not-OK-ness with the antenna manifests itself) as return loss - the decibel relationship of forward power to reflected power. For instance, sending 100 watts and seeing 10 watts reflected back to you (10%, or 1/10) is -10 dB. If you send 100 watts and see 25 watts coming back to you that's -6 dB, and so forth and so on. The minus is often omitted on the grounds that it's impossible to have negative return loss (that would be gain!). Unsurprisingly, the same method of expressing imperfections is employed by telecommunications fiber people.

Too much reflected power results in heat build up and kills components. Too-high voltage from reactance in the system is a more immediate killer. Modern amplifiers tend to have safety circuilts to try to save themselves from this sort of thing, but that's not perfect. You want to have return loss that is high enough (low enough reflected power) that it is well within your transmitter's specifications.

Most modern transceivers that do not have an internal tuner are specifiec to handle an SWR of up to 2:1, which coincidentally is a hair under 10 dB return loss. But that doesn't have infinite significant digits nor is the safety monitor something that gets particularly calibrated. On practical antennas, wind blowing will move the SWR (or reflected power) around a little bit too, so you'd do well to target 1.8:1 SWR (11 dB return loss) or better.

In the interests of having a handy reference, here's a table with SWR, return loss (dB) and reflected power as a percent of forward power. Values are approximate.

SWR	RL (dB)	Power Back (%)
6:1	2.92	51.1
5:1	3.52	44.5
4:1	4.44	36.0
3:1	6.02	25.0
2:1	9.54	11.2
1.9:1	10.16	9.6
1.8:1	10.88	8.2
1.7:1	11.73	6.7
1.6:1	12.74	5.3
1.5:1	13.97	4.0
1.4:1	15.56	2.8
1.3	17.70	1.7
1.2	20.83	0.83
1.1	26.44	0.22

Further refinement on how one thinks about reflected power can be had by reading Walt Maxwell's Reflections series of articles - but if you don't have the time, keep in mind that the sole real goal here is to keep your finals happy. There's no fraction in hunting for a perfect match. The first law of thermodynamics isn't violated, Power isn't "lost" in any significant way for anything even remotely resembling a decent match if you've bought coax with decent loss characteristics for the band you're using and the length of the cable run. .