What It Means
Corpus is the total value of an investment at a given point in time, your own contributions plus everything they've earned, added together. When someone talks about their "SIP corpus" or "retirement corpus," they mean this one combined number, contributions and returns lumped in together, not the profit portion alone.
Example
Say you've been running a SIP for several years and have put in ₹6,00,000 in total. Your latest statement shows the investment is now worth ₹9,50,000. That ₹9,50,000 is your corpus. Split apart, it's ₹6,00,000 of your own contributions plus ₹3,50,000 of returns earned along the way.
Warning
A common mix-up is treating the whole corpus as profit. In the example above, only ₹3,50,000 of the ₹9,50,000 corpus is actual gain, the rest is money you put in yourself. Always separate contributions from returns before judging how well an investment has actually performed.