What does Porphyritic mean ?

Porphyritic is commonly applied to igneous rocks and relates to their bi-modal grain size distribution; meaning 2 main grain sizes usually one larger than the other. The occurrence of this is down to the different cooling rates and melting temperatures; at degree level we go on about the solidus and liquidus.
In a granite for instance, you will usually have K-feldspar crystals larger than the plagioclase feldspar crystals. This is due to K-feldspar having a higher melting point, hence starts to crystallise faster and due to it being a crystal that crystallises while the rest of the melt is still molten it has lots of room, thus has a nigh growth rate and nucleation rate. The plagioclase crystals in comparison have a lower melting point, and thus starts to crystallise later, when more of the melt has crystallises into biotite, Muscovite and quartz (etc). So these plagioclase crystals have less room to grow and thus are smaller. At degree we learn this in more depth in relation to energy at crystal boundaries and more reasons for the grain size difference.