Shortcuts is Apple's new version of Workflow that launched with iOS 12. It is a productivity tool that does what its name implies - each shortcut is basically a program or 'workflow' that runs through a set of steps to produce a desired result. For example, Home ETA is a navigation shortcut that uses Maps and Messages to send someone (pre-determined by you) your arrival time with a single tap.
Apple's Shortcuts app offers hundreds of pre-written algorithms designed to accomplish tasks that would normally take many steps in just one or a few steps. For example, the Home ETA shortcut uses Maps and Messages to find your location, find a route home, estimate the time that route takes and text that time to whoever you set up the shortcut to send it to, all with a single tap.
If you have added shortcut icons to your Home Screen and want to change them, just open the Shortcuts app, go to your Library and tap on the "..." button in the top right of the shortcut you want to change.
If you want to share your shortcut with others (or any of the pre-made shortcuts), simply open the Shortcuts app and tap on the three dot button in the top right corner of any shortcut in your Library.