Transaction costs represent the total costs of buying and selling investments within the fund over the year. Oddly enough, in some cases they're shown as a negative number e.g. -0.01%. That's because the fund manager has offset the buying/selling costs using other pricing mechanisms used within the fund.
Like ongoing charges, you'll be able to see the transaction costs of a fund you're investing in by looking to its key investor information document (KIID) or when you place your order in the app - on the review your investment screen. Whatever the transaction cost is, it'll simply be taken from your investment in the fund itself (so you don't need to do a thing!).