What does this equation mean?

Tides are the response of a rotating, irregularly shaped ocean to the gravitational forcing of the Moon and Sun. The forcing is not one periodic term but many, because the relevant astronomical cycles — lunar day, solar day, lunar month, the inclination of the orbits, the precession of the lunar nodes — beat against one another. The insight that made mechanical prediction possible was that the local water level can be written as a constant plus a sum of sinusoids whose frequencies are fixed by astronomy and whose amplitudes and phases are fixed by the harbour: $h(t) = H_0 + \sum_{n=1}^{N} A_n \cos(\omega_n t + \phi_n)$. Here $\omega_n$ is a known astronomical angular frequency, while $A_n$ and $\phi_n$ are constituents determined by fitting the expression to a record of observed water levels at that specific place. The division of labour is the whole trick. The frequencies are universal and can be built into gearing once. The amplitudes and phases are local and must be adjustable.