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, it was considered paradoxical that the 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 many particle-like attributes, e.g. energy given by $E = h\nu$. This relation is exactly what Einstein used to resolve the paradox: only a photon with a high enough frequency $\nu$ has enough energy 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: http://physics.stackexchange.com/a/2288/16660. But I would make it into a different question if you want more information.
$^1$ Although you hear people call them that, it's misleading to speak of particles because it suggests light consists of particles. Physicists are careful to use the term quanta instead of particles but what exactly those quanta are, no one can satisfyingly answer. All we know is that they exhibit both particle- and wave-like behaviour.