Yoga is an ancient practice that involves physical, mental and espiritual activity. It has different schools (Hatha, Bikram, Raja, etc) and it is practised in the context of different religions too (Bhuddism, Hindiusm for example). This practice that also involves meditation originated in the 15th century and became popular in the western world in the 19-20th century. In 2016 UNESCO declared Yoga practise Intangible cultural heritage. As you can start to guess yoga is more than simple stretching exercises.
Yoga is a flow of poses that correctly done and sequenced increase muscular tone, mental awakening and coordination. It also improves flexibility, your posture, lowers the resting heart rate, increases endurance, and LOWERS BLOOD PRESSURE.
Not long ago, the American Heart Assoc. Professional Education Committee run a large study showing evidence of the benefits of yoga. They suggest that people with Blood pressure higher than 120/80 should consider trials of alternative approaches as adjuvant methods to help lower blood pressure.
Yoga is a gentle practise or a hard one. You make your practice. There are several videos you can watch to start your practice at home today, such us this one here