Short-term investments is spare cash put to work instead of sitting in the bank: government and corporate bonds, term deposits, and similar holdings that mature within a year or can be sold quickly. Functionally it is the second shelf of the cash cupboard, slightly less instant than cash and equivalents, slightly better paid.
Read the two lines together: their sum is the company's real liquid firepower, which is why analysts usually just add them up (and why net debt subtracts both). A company holding billions here is not investing in its business, it is storing value while deciding what to do, so the interest income line is largely this money earning its keep.