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Underlying refers to different securities in the case of derivatives and in the case of equities.
Underlying in the case of derivatives (such as option) is the security, usually a stock, which must be delivered when the option contract is exercised.
Underlying in the case of equities is the common stock that must be delivered when a convertible preferred share is converted to common stock or when a convertible bond is converted to a common stock, or when a warrant is exercised.
The main factor that determines the price of derivatives, convertibles or warrants is the price of underlying security, and therefore, change in an underlying brings the change in the price of an asset that underlying is linked to.