I mean it’s an app called measure. It looks like this:
It should be been installed by default, but if you restored a backup from a previous phone it’s possible that you deleted it at some point. If that’s the case you can find it in the App Store to reinstall it:
Turn your iPhone or iPad into a tape measure with Measure. Measure enables you to quickly gauge the size of real objects, a person’s height, and it can automatically provide the dimensions of rectangular objects. Features • Draw lines and estimate their length in both horizontal and vertical...