what is K$ in Mathematica?

Sometimes, when I try to evaluate something I get a K$ followed by some
number, what is this K$ ? I Searched entire online help and wolfram website
and I couldn't find anything on this K$. Here is an example:

In[5]:=
DSolve[Derivative[1][f][x]*
Derivative[1][g][x] ==
D[f[x]*g[x], x], f[x], x]


Out[5]=
{{f[x] -> E^Integrate[
-(Derivative[1][g][
K$74]/(g[K$74] -
Derivative[1][g][
K$74])), {K$74, 1,
x}]*C[1]}}




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