From the documentation:
executable
Java executable from which the core dump was produced.
core
core file for which the memory map is to be printed.
I agree, that is a bit terse...
The core is the core dump to get the heap from, and the executable is the Java executable the app that produced the core dump was run with
And this question provides an example of using it:
jmap -dump:format=b,file=dump.hprof /usr/java/jdk1.6.0_16/bin/java core.dump.2878