There are usually two numbers cited in ratings.
An example: The Daniel Avery Show 22.5/40
From what I understand, the first number is the "rating," or estimated number of people watching. If one rating point equals 1 million viewers, they estimated 22.5 million people watched The Daniel Avery Show. The number equated with one rating point has gone up and down over the years (typically between 700k and 1.1 million) but I'll call it 1 million just to make the math easy.
The second number, not always cited but also important for analysis is the "share" of the available audience. In this example, out of all the households who could have been or were watching television, the share of TV's tuned to TDAS was 40 (implying the other 60% were pathetic losers watching something else).
Ratings and share numbers kept going down as options for viewing increased, as more channels kept drawing potential viewers away from the traditional networks. In 1963, TDAS would have been #16 for the week, but in 2024 it would not only be #1, it would be several million viewers ahead of the #2 show.