On the Trade page, there are three different prices under Contract Details. The top one (19505.5) represents the Last Price, the left one (19511.09) is the Index Price, and the last number on the right (19510.86) is the Mark Price:
Last Price is the last traded price on BitMEX. This is the price your contracts are bought and sold at, and it is subject to price swings disconnected from the price of the underlying index.
Mark Price is a derivation of the Index Price used to avoid price manipulation, calculate position margin, and trigger liquidations. It's the Index plus Funding Basis. You can find more details on Mark Price on Fair Price Marking page.