Do you mean that vertical lines appears sometimes on your screen when you touch it? It could be a problem with your screen or software issues...
Make sure your iOS version is up to date (Settings > General > Software Update).
If the screen problem occurs again, try pressing in the same time the Power + volume down buttons until the Apple logo appears.
You can also try reinstalling a new copy of iOS using iTunes. First back up your iPhone, then restore it by clicking Restore iPhone. Once iOS is reinstalled and your iPhone is restored, restore your backup.
In case the problem persists, your screen may need to be replaced. You can contact Apple Customer Support or go to a certified repair center.