HDR is an abbreviation for high dynamic range.  There are numerous sources of information on this, and some fairly complex ways of achieving the effect.  The easiest cheat is to adjust the highlights and shadows in the editing menu of your Apple device’s Photos app.

Photos on Mac

Simply launch your Photos app, locate the slider icon in the upper right next to Details (on your Mac), or at the bottom of your screen (iPhone or iPad)

Photos on iPhone

This will open the photo editing menu, and give you all sorts of options to make your photo look worse.  Don’t worry, this is not destructive.  They provide a nice little button that allows you to “Revert to Original”.  Once open you can click on the “Adjust” button that looks like a little knob. That same knob icon appears on the bottom of the screen on the iPhone.  On the Mac, you will see the sliders to adjust the highlights and shadows.

Sliding the highlights to the left brings the brightest part of the picture down so the highlights aren’t blown out.  Sliding the shadows to the right brightens the dark shadowed areas to create the look of an HDR photo.  You can even rescue some slightly incorrectly exposed photos with this.  Just don’t take it too far.  As you slide the adjustments you will notice the changes and you want to stop short of making it look fake.  Less is more here.  Play with it and see what you think!  Here’s an extreme example:

Before

After