Overtraining can also be labeled as "under-recovery."
It can be described as too much training for your ability to recover between workouts in a weekly, monthly, or quarterly period (micro or macro-cycle).
Your ability to recover can be hindered by lack of or poor sleep quality, hydration, and nutrition. If you are low on calories or vitamins/minerals/protein your body cannot replenish itself as fast. If you cannot breakdown such nutrients for any reason, same effect.
High stress levels contribute to under-recovery.
Massage, spa/sauna, contrast bathing/showers, ice baths can aid in allowing the muscles to recover.
In this day and age, many youths and athletes have too much going on that it can be an issue. Multiple sport/activities, lack of hydration/nutrition or proper rest.