"Last Traded Price" is the price at which the last order in the market was carried out at. For example, if the last trade that went through for BTC/EUR was 6950, the Last Traded Price is 6950. Last Traded Price is purely historical and is not the price that a market order will be executed at.

The “Index price” for a market is the price of the corresponding Index provided by CF Benchmarks, determined from aggregated data from multiple exchanges. In essence, an index aggregates open orders observed in several exchanges and trading venues in real time to derive an instantaneous price for the demand to buy or sell a particular asset against another asset. Index prices help to ensure that in times of high volatility, orders are triggered based on the prevailing price in the market at the time and not only on the Kraken exchange. 

"Market price" for an order means the lowest current ask price (for buy orders), or the highest current bid price (for sell orders). Market price is essentially the best offer on the order book, which will be different for buyers and sellers, since the best offer for buyers is the lowest sell order on the book and the best offer for sellers is the highest buy offer on the book.

The "best average market price" for a market order means the (weighted) average price of the best current asks or bids that can fill the order. This is important for understanding how market orders fill. Typically a market order will be filled by several opposing orders on the book. A market buy order will be filled by several of the lowest asks on the book and a market sell order will be filled by several of the highest bids on the book. Hence, the average fill price of a market order will be the best average market price - a weighted average of the different orders at different prices that filled the market order (weighted according to the size of the filling orders).

The limit price is the price at which you want a limit order to be fulfilled or better.

For Futures markets, the mark price is the mid price of Order Book bounded by a range defined by the CME CF Index Price with anti-manipulation coefficient. If selected as the trigger signal, the order trigger will activate when the mark price price reaches or surpasses your trigger price. Note: this price is also used to value positions and determine liquidations.