2.4GHz = 2,400,000,000 Hz = It does SOMETHING, 2,400,000,000 times every second.
How much it actually does every time it does something depends on the CPU itself. Athlon processors do more every time it does something than, say, the Intel P4's. So, for EQUAL performance, Athlon CPU's can run at a lower frequency than P4's.
And the new Intel Core 2 Duo CPU's do more every time it does something than the AMD Athlon CPU's.