HDR is a clever piece of trickery that takes the best of two images that are taken at the same time with one press. What is does is takes the best contrast from the dark areas and the best contrast from the light areas and joins them together so you get one perfect image for contrast, brightness and detail. If yours has a line through it, it means it is switched off. Better to have it on.
But what you really need to understand is how and when to turn HDR on. It all depends on the situation, so here are some things to keep in mind.
HDR will improve many outdoor and landscapes scenes. Because these settings often have both dark and light areas, you will often lose details. Turn on HDR to compensate.
HDR is great for making the most of dim lighting. Just make sure you turn off the flash. It won’t work well with the longer exposure.
Because it is taking several different images, HDR is slower. So if you are capturing a moving object, or you are taking several photos in quick succession, you should probably turn HDR off.
HDR will eliminate shadowy or washed out areas. So if you are trying to create a certain mood, or photograph a silhouette, you should turn HDR off.
Beyond that, experiment! Now that you understand what HDR does, it’s up to you to see how it affects your photos. Some photographers make their whole living shooting HDR images, while others never use it. https://learn.corel.com/blog/hdr-iphone-camera