Let me start with what continental drift is.
It is the movement of continents on the earth's
surface. Which way they move depends on the
plate tectonics that they're next to -- divergent,
convergent or transform plates). In California
you're next to the big San Andreas Fault, which
makes California move (or drift) away from the
rest of the USA.
To study the continental drift of California, we
use satellites and GPS to measure how much it
moves every year.
But how about the fossils Fossils are
organisms, usually animals or plants, that have
died and been buried for a long time. So deep
underground, the fossils will exist there no
matter if the land is moving due to continental
drift. The continents will drift whether or not
there are fossils so we can say that the
fossils will not influence continental drift.
But we can use fossils to track how the
continents have moved. Let's fast forward 50
million years when California has drifted away
from the USA. If there is a plant that only grows
in California and Nevada, we will only find
fossils of this plant in California and Nevada. We
can then trace back that California used to be
attached next to Nevada because of the where we've
found the plant fossil.