The Filling option can take a list of rules that describes the filling. This one says for example to only fill from the first function to the bottom:
Plot[{Sin[x], 0.1, 0.3}, {x, 0, 10}, Filling -> {1 -> Bottom}]
If you are not comfortable with the Filling syntax, make multiple Plots and put them together with Show:
p1=Plot[Sin[x], {x, 0, 10}, Filling->Bottom];
p2=Plot[{0,1,0.3}, {x, 0, 10}];
Show[p1,p2]
