When you're working in Photoshop, it's easy to stare and stare at an image for hours and somehow still miss some little detail; we get tunnel vision, our eyes get tired, etc. This is when check layers can be an incredibly useful way to ensure you don't miss a thing.
Coming to you from Unmesh Dinda of PiXimperfect, this video will teach you all about different check layers and their uses. The idea of a check layer is simple: take advantage of Photoshop's image manipulation abilities to either focus in on a single property or vastly increase the visibility of detail in an image. This is done simply to assist you in ensuring that you haven't missed any subtle issues and that everything is as you'd like. They can also be a huge time-saver, as you'll be able to quickly evaluate an image and/or spot any inconsistencies instead of staring closely at each part of the photo. I personally use solar curves quite a bit in my work; it's easy for me to sometimes miss a speck of dirt on a client's clothing or the like, and the stark relief into which the layer throws the image makes any such imperfection perfectly clear and easily fixed. Give them a try in your work!