The answer would be: the contribution an individual makes to the gene pool of the next generation relative to the contribution of another individual
Relative fitness is the capability for an organism to reproduce and crate viable offspring (offspring live to maturity and which can reproduce themselves). It is the survival rate relative to the maximum survival rate with other population.
For example if organism A ends up finding a mate and have babies and organism B does not this means that organism A has more fitness biologically than organism B.