Keynote has image masking built in. Masking allows you to more or less shape a picture in a frame. A simple example would be showing a portion of a photo in a circle rather than as a square.
This is easy to do and can help make a presentation look a little more polished.
A more advanced version is masking an image with text. Here's a great example of this technique:
(Fun side note: Disney was hit with a copyright suit for this string of promo posters.)
You can't do this in Keynote on iOS, though. It's not part of the text formatting settings you would need.
Mask text in Keynote
You can't mask text natively in Keynote. But, you can use an image of text whipped up in Pages (or similar) to create the same effect. Here's the final result:
First, make some big, bold text. I did this in Pages because the font choices are easier to use. When you have your word, take a screen shot and crop it down.
Add your base image and the text to the Keynote slide with the text on top. The, select the text screenshot and use Instant Alpha in the format menu to remove the inside of the letters.
After removing your text, you should be able to see your image in the empty space. Crop the image down (double-tap) so it's the same size as the text layer.
Finally, it's a good idea to lock the text layer and the image together in a Group so they can be positioned as a single object. Tap your text and while you're holding, tap the image in the transparent area. This will select both objects and bring up a menu. Select Group to lock them together.
Hey presto, you now have a masked image.
Just don't land in hot water like Disney.