A CSM (Customer Success Manager) score is a manual score that you and your team can set. Many use it to indicate the status of the relationship between their company to the customer.
It is an additional factor you can use to help you better understand the account and which actions you need to take.
For example, it could be that the customer is really using your service so their activity data looks great. But, they could be unhappy and express that in a meeting or conversation and this is important to take note of. This is what the CSM score can be used for.
CSM scores can also be used in your customer health score!