In simple words
In Vedic astrology your life moves through nine long chapters called dashas, always in the same order. Which one you start in depends on your Moon at birth.
The Mercury dasha lasts 17 years. While you are in it, Mercury's themes colour those years. Your chart page in Vedic mode shows which dasha you are in now, with dates.
Themes
The Mercury period lasts seventeen years and highlights intellect, communication, commerce and adaptability. It is often a time of study, writing, business and building networks. Curiosity is strong, and the skills you sharpen now often shape your work for many years afterward.
What it supports
Education, writing, trade, analysis, technology, friendships and skilful communication.
What it asks of you
Mercury asks you to use your cleverness with integrity. Focus your many interests into a few skills you master.