Traffic data is mainly:
Information to identify the user of your Wi-Fi network: IP address, phone number, email address, etc.
Information about the devices that users  use to be connected to the Wi-Fi: MAC address, etc.
Login logs: date, time and duration of each connection
These data must be kept for 12 months. If one of your Wi-Fi users commits an offense on it, the judicial authorities will require that you provide them with this data.