The ‘SwitchToIframe’ action is essential when encountering IFrames on your site. If you aren’t familiar with this, an Iframe is essentially a HTML document embedded inside another HTML document on a website. Common examples for when you might encounter an Iframe are through advertisements and payment input fields.
Once you have identified the Iframe, you firstly have to find the ID, Class or identifier to make sure you are entering the right element. Enter the selector into the ‘SwitchToIframe’ action and then proceed with the interaction you want to perform within that Iframe.
Here is an example using the ‘Iframe’ within a journey:

As you can see, the journey clicks onto the payment page and enters the Iframe to enter a card number within the input field. It is worth noting that once you have performed the action you have to make sure to exit the Iframe you have entered. If the ‘SwitchToDefault’ isn’t added immediately after the action, the journey will likely fail. Here is a link to the documentation on SwitchToDefault.