As essential as layer masks in Photoshop, blending modes allow you to control the way the program combines and applies different layers, effect, images, and adjustments to your image. While it's not critical to memorize each blending mode's manner of operation, knowing how each category works is essential to getting the results you want. This great video will give you a quick course in just that.
Coming to you from Jesús Ramirez of the Photoshop Training Channel, this helpful video will give you a good introduction to blending modes in Photoshop. Simply put, when you have two layers (whether they're two images, an adjustment layer on top of a photo, etc.), you need to tell Photoshop how to combine them to output the final image. Blending modes, at their core, are simply mathematical equations that tell Photoshop how to do just that to produce a result. You certainly don't need to know these equations or even what every mode does exactly, but it's important to understand the different categories and how they affect your work, as you'll find them indispensable the deeper you get into post-processing.
And if you'd really like to dive into blending modes, check out this video, also from PTC:
Now, get into Photoshop and start creating!