What does h mean in algebra?

In algebra, the h stands for an unknown number.
a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, and z can all stand for an unknown number.
For example: f-1=2
f is unknown and can be solved for.
However here is another example: f-1=x+2
In this case, f and x do NOT equal the same thing.