Adjustments, or spinal manipulations as they're sometimes referred to, is the minor movement by the chiropractor of vertebrae in the spine. The objective of this movement is to realign vertebrae that have moved out of place (i.e. they are not articulating with surrounding structures properly). Similarly, for those chiropractors who do additional training with extremities (e.g. shoulder, knee, foot), adjustments can be performed on peripheral joints in order to correct for existing aberrant motion or subluxation there.
When vertebrae are out of place, the central nervous system becomes involved and it can have an overall systemic effect. This impairs the ability of your brain to communicate effectively with your body. Simply stated, without proper alignment we cannot function optimally.
An adjustment is specific pressure applied from the chiropractor utilizing their hands or an instrument to move a vertebrae or joint back into place. This happens with a quick movement and is most commonly without discomfort. You may hear a noise that sounds like you're cracking your knuckles referred to as joint cavitation. Noise is not necessary to achieve the desired result; however, it is common depending on the chiropractic technique being employed. Luckily, this is almost always a pleasurable experience, not a painful one!
Overall, adjustments are an excellent way to keep the body functioning at its highest level. When the body is in alignment, the body is able to respond and perform as it was designed to. Think less about pain and more about optimal function.