Long term assets such as mortgages are illiquid in the sense that they cannot easily be sold in financial markets for cash. Securitisation provides a mechanism for mortgage lenders to realise the value of mortgage assets prior to their maturity. A pool of mortgages and the cash flow they will generate form the underlying assets for a sale of tradeable securities, such as bonds, to investors.
Such transactions are called securitisations because payments on the bonds bought by investors are dependent on the cashflows from, and are secured over, the illiquid assets. The illiquid assets can be said to have been converted or "securitised" into liquid assets (bonds).