What does ARR stand for?

ARR stands for Annual Recurring Revenue, and is just like MRR, but uses one year as a normalization period. When only annual subscriptions are sold and ARR is as a main metric, there are several variations like with MRR.

However, when your main metric is MRR, we can use following formulas:
When estimating future ARR for a single month: MRR x 12
When reporting past ARR in annual report: Sum of MRRs from past 12 months

See also MRRRun RateRecurring Revenue