That code is a 1970s way of displaying the strength of a signal over time, effectively as a number of standard deviation from the baseline.
Clinically,
the signal intensity was measured as signal-to-noise ratio, with the noise (or baseline) averaged over the previous few minutes. The signal was sampled for 10 seconds and then processed by the computer, which took 2 seconds. Therefore, every 12 seconds the result for each frequency channel was output on the printout as a single character, representing the 10-second average intensity, minus the baseline, expressed as a dimensionless multiple of the signal's standard deviation.
In the chosen alphanumeric measuring system, a space character denotes an intensity between 0 and 1, that is between baseline and one standard deviation above it. The numbers 1 to 9 denote the correspondingly numbered intensities (from 1 to 9); intensities of 10 and above are indicated by a letter: "A" corresponds to intensities between 10 and 11, "B" to 11 to 12, and so on. The Wow! signal's highest measured value was "U" (an intensity between 30 and 31), that is thirty standard deviations above background noise.