Reset also known as fixing is a generic concept in the financial market, meaning the determination and recording of a reference rate, usually in order to calculate the settlement value of a periodic payment schedule between two parties.
Resets are most commonly used in Interest rate swaps, to determine the value of the floating rate payment for each period. The parties will have agreed a source for the reference rate (usually a named screen on an information vendors system, though any public domain source will do, such as a newspaper, government publication &c). Fixing involves looking up the reference value on the agreed date, recording, then computing a payment based on the rate.
Fixing can often change the value of a financial instrument, and can be difficult to encode in the software models used to price such instruments. Other examples of fixing are in average rate options, where the underlying price of the option is an average of some kind, also in -ONIA type swaps, where periodic payments are made on the basis of average overnight interest rates.