Simply put, razor burn is when you remove too much skin when shaving. Applying too much pressure, shaving over the same area too many times, and/or using product that doesn't form an adequate barrier between the blade and your face are all causes of razor burn.
If you don't replace your blades frequently enough, you may be pressing harder or going over the same area multiple times to try to get a closer shave. Shaving against the grain also leads you to apply more pressure since there is more resistance. Both of these techniques will cause razor burn.
Remember this: The goal is to get as close a shave as possible with as few strokes as possible. This is why we recommend multi-blade razors. Single-blade razors are less efficient since you must go over the same area more times to get the same result as with a two- or three-blade razor, usually resulting in razor burn. A poorly weighted and balanced razor could be causing you to have ineffective, uneven strokes also. You should use light, easy, even strokes and your shaving product should lift your hairs and provide a protective barrier between your face and your blade.
Using a preshave oil will provide an extra barrier that allows the blades to glide better. After shaving, be sure to use a Post Shave product such as Maxwell's Apothecary Razor Relief Aftershave Balm. Also, moisturize your face before you sleep since dry skin can also lead to razor burn.