Monthly Recurring Revenue (MRR) is the predictable recurring revenue earned from subscriptions in a specific period. As the name indicates, MRR is calculated for a monthly "duration". The MRR amount shown on the dashboard is a total of all your subscriptions' MRR.
In a monthly plan, if you have 10 customers each paying you $100 per month, then the MRR is $1,000.
In a yearly plan, if $1,200 is paid annually, then the MRR is normalized as $100 per month.