Customers up for Renewal tracks the count of customers with contracts that are ending in a period, and need to be renewed. Most SaaS companies either have customers pay month-to-month, or have them sign long term contracts (typical 1 year). For customers that sign longer term contracts, when the contract expires they have to be renewed for another term.
Customers up for Renewal = count of customers with expiring contracts
Customers up for Renewal is the denominator when calculating renewal log retention. The numerator is the customers renewed: the number of customers with expiring contracts who sign new contracts. Tracking how often customers churn at renewal is key to understanding if customers are getting consistent value from the product.