They are constant values used as an input to a UGen.
The “constants” in your example are Control defaults, which are encoded into the SynthDef’s list of controls.
The expression Out.ar(out, (SinOsc.ar(freq) * amp).dup) happens to use no constants at all! All of the UGen inputs here are expressions.
Oh wait… except one input! SinOsc has a phase input, which isn’t specified, so it reverts to the default = 0… and the constants collection does include a 0.
If you add, for instance, an EnvGen, the Env definition is extremely likely to include at least constants 0 (initial and final level) and 1 (peak level), perhaps others. Then you’ll have more constants to look at.
an input-spec is :
int32 - index of unit generator or -1 for a constant
if (unit generator index == -1) : int32 - index of constant else : int32 - index of unit generator output