I think you're confusing your AE Lingo.
It sounds as if you're using "mask" to refer to "track matte". I recommend opening AE's online help. If you search for the words track matte, you'll reveal some very useful information.
Don't worry about the version of AE Help you get. Track mattes haven't changed in their function for over a decade.
Two options here. Option #1:
- Create the text layer
- Use the Mask tool to create a mask over the text layer.
- Edit either the Position property of the Text by adding a Text Animator, or animate the position of the mask by enabling keyframes on the Mask Path.
- Create your text layer
- Create a solid layer above that is either the size that you want for your mask or use the mask tools to masked the solid layer to the size required
- Select the text layer, reveal the track matte column (F4) and set the solid above as an alpha track matte for the layer.
- Animate the text layer.
Make sure that you've gone through the Getting Started materials from Todd.
got it. Thank you!