Underlying asset is a term from derivatives trading. For example, in the case of a Microsoft stock option, the underlying asset is Microsoft stock. In the case of gold options, the underlying asset is gold. Price movements of options are derived from price movements of the underlying assets – hence the term derivatives.
It is possible to have underlying assets which are themselves derivatives; for example, options on gold futures. In this case the gold future is the underlying asset, while the gold future is also a derivative of gold.