What is the meaning of stress?

a state of mental or emotional strain or tension resulting from adverse or very demanding circumstances.

Explanation:

"he's obviously under a lot of stress"

"Stressed and tired, she'd been too distracted by thoughts of what Jonny was plotting to grab coffee on the way in"

these are common warning signs of overtraining syndrome:

washed-out feeling, tired, drained, lack of energy

mild leg soreness, general aches, and pains

pain in muscles and joints

sudden drop in performance

insomnia

headaches

decreased immunity (increased number of colds, and sore throats)

decrease in training capacity/intensity

moodiness and irritability

depression

loss of enthusiasm for the sport

decreased appetite

increased incidence of injuries.

a compulsive need to exercise