Shopify defines this as:
Monthly Recurring Revenue, or MRR, is calculated by multiplying the number of merchants by the average monthly subscription plan fee in effect on the last day of that period and is used by management as a directional indicator of subscription solutions revenue going forward assuming merchants maintain their subscription plan the following month.
Last reported quarter 2018 Q4 it was $40.90 million, up by 37% year-over-year from $29.90 million. From $101.10 million in 2017 it increased by 45% to $146.60 million in 2018.
Source: Shopify Quarterly Results.