The underlying assets giving value to a company, investment or loan. The asset base is not fixed, it will appreciate or depreciate according to market forces. Lenders use physical assets as a guarantee that at least a portion of money lent can be recouped through the sale of the backed asset in the case that the loan itself cannot be repaid.
The value of a home might increase or decrease over time, affecting the underlying collateral in a mortgage. Similarly, the price of a commodity used as the asset base of derivative can also increase or decrease rapidly, changing the price that investors are willing to pay for it. Examples of asset bases include a home (for a mortgage) and factory equipment (business loan). A derivative would "derive" its value from an underlying asset.
The company opened its business to the public to expand its asset base of $2 billion.