The CAPA project is a means to document the different corrective and preventive actions that your company is implementing.
Corrective actions are actions that are taken to avoid recurrence of an issue. Simply put, you want to avoid things to go wrong again.
These actions are documented using CA items.
Preventive actions are actions that are taken to avoid occurrence of an issue. This means you detected that something could go wrong (although it didn't yet) and you want to avoid this.
These actions are documented using PA items.