When creating a web link field in the App Builder, if you select the Button style, you have the option to set a Button Label. This is what will appear inside the button when a user is filling out your App on their mobile device. It will not appear if you select the Link style (in that instance, the link will be the Field Label).
By default, the Button Label will display as "Tap here...".
In the App Builder, you will find this setting in Field Settings area:
And on mobile:
Button Labels are particularly useful for giving users an idea of where they will be taken when clicking the link. Like Field Labels, these should be concise, but informative.