A limit order is a type of order where you can buy or sell an investment at a price you specify, or better! You decide on your desired variables when you place your limit order and then let Standard or Plus do all the hard work for you!
For example, you would place a buy limit at a price lower than the current market value so that if the stock price fell, your limit order would kick in and attempt to buy the shares you wanted at a price you're happy to pay for them.
On the flip side, you would place a sell limit at a higher price than the current market value so that once the stock price increased enough to provide you with whatever you consider a decent return on your investment, your limit order would kick in and attempt to sell your investment for you.
When entering a cash amount for a buy limit, this will also cover stamp duty. You may find that the amount of shares that you receive is lower than expected.
For example; if you select a limit price of £5, and reserve £10 cash in order to receive 2 shares, you may actually only receive 1 share due to stamp duty being deducted.