What is the difference between absolute age and relative age of fossils?

One has an explicit date and the other is relative
An absolute age is one determined usually by mass-spectrometry where an isotope is measured and then an age can be calculated (a very very basic explanation). So in the end you can say this fossil is 50 thousand years old (always with an associated uncertainty). We can absolute date materials but it will always have an uncertainty range we can never know the age with infinite precision.
Relative dating is like looking at a multi-layered cake. The base layer must come first then the second and then the third. So I can say the second layer is younger than the first and older than the third but I have no idea whether the cake is 5 minutes or 5 million years old. Relative dating is used commonly when looking at the relative order of geological events.
What can complicate relative dating is when the strata is not the right way up! Sometimes beds of rock can turn over the other way so be very cautious when relatively dating rocks!