the main boom is the long wooden, or aluminum pole that the sails are attached to.There is one for each sail, except in a gaff rigged sail boat.They have two, the top one is called a gaff and the bottom one a boom.
a "reef" refers to making the sail smaller because of strong winds that could make the sail blow out. If it's made smaller there is less pressure on it.
Picture a big square sail full of wind. It is straining under the pressure. Now picture a row of little holes in the sail that runs parallel to the boom. Now picture pieces of small rope which get drawn through the holes and tied down to the boom. That is a reefed sail.

Not being a sailor, you would have no need to know about such things.

My first wife and I were caught in a fierce gale in Penobscot Bay, and it blew so hard that a new jib ripped right up the middle. There were no reef points in the jib, so, no way to make it smaller.
I wish my Mum had taught me songs like yours did. She had very little to sing about.