The article quoted actually starts by saying :- "This article is a preprint and has not been certified by peer review ... It reports new medical research that has yet to be evaluated and so should not be used to guide clinical practice."

Myocarditis is inflammation of the heart muscle, usually caused by a virus. Depending on the cause and severity, symptoms and possible problems can range from mild ( most recover ) to life-threatening heart failure.

The authors reviewed reports among adolescents ages 12-17 who received mRNA vaccination against COVID-19. 257 "cardiac adverse events" (CAE) were identified.

"Rates per million following dose 2 among males were 162.2 (ages 12-15) and 94.0 (ages 16-17); among females, rates were 13.0 and 13.4 per million, respectively. For boys 12-15 without medical comorbidities receiving their second mRNA vaccination dose, the rate of CAE is 3.7-6.1 times higher than their 120-day COVID-19 hospitalization risk as of August 21, 2021 (7-day hospitalizations 1.5/100k population) and 2.6-4.3-fold higher at times of high weekly hospitalization risk (2.1/100k), such as during January 2021. For boys 16-17 without medical comorbidities, the rate of CAE is currently 2.1-3.5 times higher than their 120-day COVID-19 hospitalization risk, and 1.5-2.5 times higher at times of high weekly COVID-19 hospitalization."

The authors conclude "Further research into the severity and long-term sequelae of post-vaccination CAE is warranted."

Once again, media reports focus on relative risks rather than absolute risks of rare events, and as such should be taken with a pinch of salt ( only one, or it will raise blood pressure ).