EXPLANATION: In order to understand this concept, take the examples of D-Glucose, D-Mannose and D-Galactose .
As can be seen in the image, D-Mannose differs from D-Glucose ONLY at the 2nd Carbon atom. Hence, it is a C-2 epimer of D-Glucose.
D-Galactose, on the other hand, differs from D-Glucose ONLY at the 4th Carbon atom. Hence, it is a C-4 epimer of D-Glucose.
D-Mannose and D-Galactose. Are they epimers of each other? Are they isomers of each other?
These are not epimers since they differ in configuration around two carbon atoms, they are simple isomers.
For complete description of Isomers, anomers and epimers follow the link