Why SWR compares the impedance of the load (usually the antenna) to the characteristic impedance of the feed line. When they match, all forward power goes into the load and SWR is 1:1; when they differ, some power reflects back and combines with the forward wave to create standing waves of voltage and current along the line. Numerically it is the ratio of the larger impedance to the smaller, so a 100 ohm antenna on 50 ohm coax gives 2:1.
Watch out The ratio of amplifier output to input power is gain, not SWR, and neither transmitter efficiency nor ground quality is what an SWR meter reads.
SWR = Standing Wave Ratio, but think 'System-to-Wire Ratio': it grades the match between line and load. 1:1 is perfect.