Leonid meteors are dust particles that have come off Comet Tempel-Tuttle. Most of this dust is still following the comet fairly closely in space. The comet takes 33 years to complete an orbit around the Sun, and planet Earth ploughs through its main dust trail when the comet returns to our vicinity every 33 years. In the years when this happens, a strong shower or storm takes place. In the years between returns of the comet, a very small number of Leonid meteors are seen in mid-November.
Some meteor showers produce about the same rate of meteors around the same date every year.
Regular annual showers happen when the dust from a comet has spread around the whole of the comet's orbit, something that takes place gradually over a long period of time. An example is the Orionids, a shower in late October each year caused by dust from Halley's Comet. The Leonids are so-called because the trails of the meteors belonging to the shower appear to radiate out from a point in the constellation Leo. But this is an effect of perspective. In reality, the meteor particles enter the atmosphere along parallel tracks from the same direction in space.