The footing is which octave an oscillator is transposed to. It can usually be toggled with a footing switch, often labelled range (or, occasionally, scale).
As close as there is to a standard, the 8' setting should play the original unmodified pitch. 16' should play an octave down, 32' two octaves down, etc, while 4' should play an octave up, 2' two octaves up, and so on.
If you ever needed an example of how synthesisers evolved from organs, this is it. A pipe organ's footing literally measures how many feet long its pipes are. Each doubling is an octave lower than the last, because that's how acoustic physics works: a pipe twice as long resonates at half the frequency. (This is why, when sampling an acoustic instrument, playing the sample back at a significantly different pitch makes it sound like the instrument making the sound is bigger or smaller.)
This terminology then made its way to electric organs, which use spinning metal wheels and electric-guitar-style pickups to achieve much the same effect as acoustic organs' pipes. When manufacturers started to make synthesisers, they based some of their terminology on electric organs. As 8' was the standard footing of pipe organs, it carried over to the standard footing of oscillators.
Before the 1980s, the footing switch was how you told the synthesiser which short set of octaves to play in. Its keyboard would likely have a limited range of just a few octaves, and the footing switch would compensate for this by selecting an appropriate octave transposition for the oscillator.
With the advent of digital control, in the form of Roland's DCB, then MIDI, musicians could finally directly choose the exact octave of each note they played, as an aspect of the sequencing rather than the sound design. This promoted 8' from a good middle ground to the unmodified note you actually asked for. But even before then, 8' was already seen as something of a standard default setting.
Naturally, some of the fun of synthesis comes from setting multiple oscillators to different octaves (to the point that even single-oscillator synthesisers sometimes have a sub-oscillator), so playing a single note might well technically be very similar to octave doubling... except, by sharing a filter and modulation, the oscillators' combined efforts come across more as a complex single note. This kind of blurring of the line between composing and sound design is part of what can make electronic music making so interesting.
- "Juno-60 service notes" Roland, Apr 1983, p. 19