The frequency of light refers to the relation $$\lambda\nu = c$$ where $\lambda$ is the wavelength, $c$ the speed of light and $\nu$ the frequency of the light (also sometimes denoted by $f$). Note that this seems to suggest that light consists of waves. However, this is misleading at best. Light also exhibits particle-like behaviour. The photo-electric effect is a typical example, since it was this experiment that led Einstein to postulate the existence of photons, "light-particles" if you will.$^1$
More in detail, when light shines on the surface of some metals, emission of electrons is observed - the electrons get "knocked out of the metal surface" so to speak. This is known as the photo-electric effect. Before Einstein's explanation in terms of photons it was considered paradoxical that this emission of electrons did not depend on the amplitude of the incident light, rather it depended on its frequency. Einstein clarified this problem by assuming the existence of photons, individual entities (quanta) with particle-like attributes, specifically with their energy given by
$$E = h\nu,$$
where $h$ is Planck's constant. This relation is exactly what Einstein used to resolve the paradox: only a photon with a sufficiently high frequency $\nu$ has enough energy $E$ to knock an electron out of the material it is incident on.
As for your second question, why $c$ is the maximum speed and if there is a relation between this and the frequency of light, this answer could be a place to start. Indeed, $c$ is called the speed of light and light in a vacuum always travels at speed $c$, but this is not a unique property of light: all massless particles must travel at speed $c$. It just so happens that we discovered this in light first, which is why $c$ is commonly called the speed of light. The group velocity of a single-frequency wave is $v_g=\lambda\nu$ and $v_g=c$ for light waves in a vacuum. As to why nothing can travel faster than $c$, this is actually a postulate of special relativity and as such we don't have a theoretical justification but we do have loads of strong experimental evidence in the form of results that match the predictions of special (and general) relativity exceptionally well. Some reading material: Travelling faster than the speed of light and Why does the (relativistic) mass of an object increase when its speed approaches that of light?.
$^1$ Although you hear people call them that, it's misleading to speak of particles because it suggests light consists of particles. A more careful term to use is quanta, and the physical nature of these quanta appears to be some blend of particle-like characteristics and wave-like properties.