What is allowed
Tourism, visiting family or friends, meetings and one-off business activities, and transit through the UK.
What is not allowed
You cannot work for a UK employer or settle in the UK. There are separate routes for that.
Long-term study is not covered either: the ETA is for short stays.
Travelling more than once
You can travel repeatedly while the authorisation is valid, but each stay is limited to 6 months, and stays cannot be chained together indefinitely.