An identifier is a name that identifies an object. This identifier can be a word, number, symbol, or a combination of these.
On a lower level (in relation to the nuts and bolts of what's going on physically), in programming, identifiers are the names of the physical location where information is stored memory. To retrieve the information stored in this location of memory, just reference the identifier. Using the identifier, you are referencing the information stored in that location of memory.
Some programming languages allow you to reference that same information by using the address of the physical location in memory. This is never done in real life programming, but it demonstrates that the identifier name and memory address are interchangeable.