It is something "kept from general knowledge or view", or "kept hidden", or "tending not to disclose information".

The definition can apply to any club, society, etc. So that makes us no different from any other social organisation in principle.

What then are the Masonic Secrets?
Here they are.
1 We work together in harmony.
2 We don't bore you with the mundane details of our meetings, so we keep them to ourselves.
3 We have existed largely unchanged since the early 1700s. Only the people have changed.
4 There are millions of us all over the world separated only by language or distance, doing the same things in only slightly differing ways.
5 We pledge ourselves to conform to the society in which we live, and treat everyone as we would want to be treated ourselves.

Every lodge has a large degree of autonomy. The Order is not ruled from the top like a commercial company. The "upper levels" are for liaison with other countries and to co-ordinate activities with other lodges, to help keep up common standards, and to co-ordinate donation of money to causes at national (country) level.

We donate money and members' time and efforts to many things in society at large, apart from our own Schools, Hospitals, Old folks homes etc. Those places also take in non-masons. Things we donate to are Medical helicopters, lifeboats, equipment for community hospitals, natural disasters. The list is endless. We also help families of deceased members.

For the purposes of this explanation I will bore you with what happens in our meetings. We introduce new members to our ways and to other members in the lodge, in procedures which are largely unchanged since the 1700s. We discuss how good/bad last month's meal was and what we should do to improve it. We discuss who is unwell and what we can do to help. We discuss events in the local community and what we can do to help. We discuss visiting other lodges. Yes, we can visit almost all the other lodges worldwide.
We have an inviolable rule that we do not discuss politics, religion or members' occupations and work. We have enough masonic business to discuss anyway.

Those are our secrets - things we have not told the world outside our own membership. I can say though we are now telling the world more about our activities.

We are proud to be members of our Order.