'Porphyritic' refers to a texture found in igneous rocks. Porphyritic rocks are those with mineral grains large enough to be seen easily with the unaided eye (phenocrysts) on a background of more finely grained rock (groundmass). This texture forms when magma starts cooling underground, forming the large crystals, and then is ejected as lava and finishes cooling very rapidly, resulting in a fine-grained texture. In some cases the magma does not actually break through the Earth's crust, and simply moves to a higher zone in the crust, resulting large mineral grains on a groundmass of smaller, but still visible grains, such as granite. The groundmass is most often basalt in volcanic rocks (AKA extrusive rocks).
These rocks give us some information on how plutonic rocks (AKA intrusive rocks) form. The minerals start to form at many individual nucleation points, and spread from there. Also, some minerals begin to crystallize before others; silicon begins to form crystals before other types of rock, forming olivine, then pyroxenes and amphiboles, then micas, and finally quartz and feldspar. This cooling also leads to a separation of dissolved metals, as they form crystals last. This often results in ore deposits including such metals as gold, copper, lead, and zinc.
A quick note on the etymology: 'Porphyritic' is the adjectival form of porphyry, that is, a rock that has a porphyrictic texture. Porphyry rocks get their name because the mixed-grain rock first identified and best-loved by the Greeks and Romans was a brownish-purple color, much prized because purple was the color of kings. The name comes from the Greek word porphyrites, meaning 'purplish'. While I love a twisted etymology, I can't help but wish that some brave geologist had had the guts to coin the word polyphaneritic.
Related rock textures include phaneritic (large grained), and aphanitic (no visible grain).