The y intercept of a function, y = f(x) can be found by substituting x = 0. Similarly, x intercept can be found by substituting y = 0. Intercept is the value at which the curve crosses that particular Axis.
In this case, demand curve is given as:
Qd = 1000 - 10 P
Q intercept is the value of Q where the demand curve crosses the Q Axis. This can be found by substituting P = 0.
In other words, Qd = 1000 - 10x0 = 1000
Similarly, for the supply curve,
Qs = -200 + 10P
P intercept is the value where supply curve intersects the P Axis. And it can be found by substituting Qs =0.
Thus, -200 + 10 P = 0
or, P = 200/10 = 20.
Hope this helps.