Short for Delayed Onset Muscle Soreness. (To everyone active in sports, it just means “I’ve done something right!”)
It usually happens 2 or more hours post-workout. More apparent when one tries new moves or unfamiliar workouts that stretches muscle groups one would usually never use.
It can last long (some might even take weeks) to recover, but can be faster with light exercises, especially those that promote blood flow and increases oxygen level in the blood, i.e. light jog or recovery run, within 24 hours.

The cause:
DOMS is still not completely understood, but it is commonly agreed that the main culprit is Microtrauma – muscle torn at the very small scale during the repeatedly contract and lengthening of muscle group.
The pain can be compound with lactic acid build up – by product of breaking down and oxidizing glucose to provide energy. (Breaking down of glucose happens when your body needs the energy ASAP for workouts like sprinting, instead of burning fat.)
DOMS is very common among those who practices HIIT (High Intensity Interval Training), Crossfit, and other conditioning workouts. (It worth the pain, apparently.)
The importance of staying hydrated and recovery workout
An important component that has always been overlooked. After workouts, your body builds up waste from burning fat, breaking down glucose and building muscle. These waste will accumulate in your body and can only be flush out through sweat and urine. Staying hydrate helps to flushing these waste out and not overwork your kidneys.
Light Exercises like light jog and stretching (Yoga!) promote blood flow and speed up recovery. It can also reduce the pain by preventing further lactic acid build up (energy still required for recovery when idle and will further breaking down glucose.)
The right diet is also important for recovery. Simple protein such as plant protein helps tremendously as it requires less breaking down, hence speed up the healing process.
This gives you one less reason to inhale that Nasi Lemak that you thought you deserve after a workout.
At this point, quality food counts more than ever.
*DOMS is also the main cause of weaklings dropping out from gyms.