A limit order is an order placed on the Luno exchange to buy or sell Bitcoin at a specific price.
A buy order will only be executed at the limit price (or lower);
A sell order will only be executed at the limit price (or higher).
If you place a buy order at a price below all of the pending sell orders, it will be pending. You are then waiting for the price of Bitcoin to decrease and for a seller to sell Bitcoin at the price of your order before your order is filled. Your buy order is considered a MAKER ORDER, because it didn’t fill immediately. If it is eventually filled, you will be charged a MAKER FEE.
If you place a buy order at the same price or at a price above any pending sell orders, and it is filled immediately, your buy order is considered a TAKER ORDER. If it is filled, you will be charged a TAKER FEE.
Similarly, the maker / taker fees also apply to sell orders.
You also have the option to create post-only limit orders on the web exchange. This ensures you won't pay taker fees: if the order would have matched, it is immediately cancelled.
Please see more information on how fees are determined on the Luno exchange.