The term KPI stands for Key Performance Indicator and it's usually a set of metrics that's most important to a particular business or an industry. When it comes to CRM, KPIs that are used most often are usually sales or activity related.
A classic example of CRM KPI is sales quota - a minimum volume of sales that each sales agent or a sales team has to meet within a given time period. Activity KPIs in CRM may include a number of phone calls or office visits made, new leads entered or deals (opportunities) initiated. In some mature industries, customer retention and cancelation reversals are most important KPIs. In helpdesk CRM, KPIs are typically build around customer satisfaction rates as well as number of inquiries processed within SLA.
It's important to understand that many CRM solutions don't use the term KPI and have these indicators available via CRM reports.
If you are looking for free CRM software with over 20 sales and marketing tools, consider Bitrix24 CRM.