MRR growth is how much your MRR has increased over some time. It is usually measured on a monthly basis e.g. your mrr growth for September 2021 might have been 10K.
Growth and Net New MRR both measure how much your MRR increased. If it decreased, then they will both be negative.
MRR is one of the most important metrics for SaaS companies. It indicates how much predictable revenue a company can expect to make. It's growth shows you how much it is increasing, and lets you forecast future MRR.
Sometimes it's easier to look at growth rate instead of raw growth. This tells you how fast a company is growing compared to it's size.
If Google increased MRR by 1 million that wouldn't be very impressive because they are a huge company. A SaaS company at 10K increasing by 20k in a year is very impressive.
MRR can be calculated by adding your new and expansion MRR together and then subtracting churned and contracted MRR. Expansion is from existing customers upgrading their account, contraction is from customers downgrading their account.
growth = new + expansion - contracted - churned
A16z recommend looking at month-on-month growth rate. This tells you how fast a company is growing each month compared to it's size. To calculate month-on-month growth rate just divide your growth by the mrr at the start of the month.
MRR growth increases as you get more customers and upsell existing customers, but decreases as customers downgrade or cancel their subscriptions.
If you are not getting many new customers then you might need to increase the number of free trials you are getting per month, or your free trial conversion rate. If you are losing a lot of customers per month you might need to decrease your churn rate.
If you are losing a lot of customers per month then you might want to decrease your churn rate. Churn has a big impact if you already have a lot of customers or MRR. If you don't already have a lot of customers then it might be more effective to focus on getting more customers each month e.g. by increasing marketing or free trial conversion rates.
If you already have a lot of paid users, then it might make sense to add upsells to your product. This is often done by scaling pricing with usage or features used. E.g. Stripe charge based on usage (number of payments) and they also have add-ons for additional features like Radar.
Upselling existing customers can be easier than selling to new customers, but the target market is smaller. If you have 100 customers, then the target market for your feature upsells is 100 users. On the other hand, upsells increase your customer Lifetime Value (LTV).
As you increase LTV then you can afford to spend more to acquire customers. So by upselling existing customers you might be able to scale paid marketing to get more new customers.
Some SaaS companies have a lot of retraction, that is existing customers reducing their bill. This can happen if users are cancelling paid features or using less of your product. I haven't seen this much before, so it's best to look at the raw numbers to make sure it is a problem first.