If statistics reporting is enabled, the information that gets submitted to the matrix.org team [according to the source code](https://github.com/matrix-org/synapse/blob/master/synapse/app/homeserver.py) is:
- your homeserver's domain name
- uptime of the homeserver program
- [Python](https://www.python.org/) version powering your homeserver
- total number of users on your home server (including bridged users)
- total number of native Matrix users on your home server
- total number of rooms on your homeserver
- total number of daily active users on your homeserver
- total number of daily active rooms on your homeserver
- total number of messages sent per day
- cache setting information
- CPU and memory statistics for the homeserver program
- database engine type and version
See [Synapse's documentation](https://github.com/matrix-org/synapse/blob/develop/docs/usage/administration/monitoring/reporting_homeserver_usage_statistics.md#available-statistics)
for a list of the individual parameters that are reported.