does a spline merely make it possible to reverse the boards?

is that the purpose of a spline?

The spline/slip tongue goes in the groove side of a milled flooring board, to make it a tongue side, to run the opposite direction.

then are splines pre made, if so, who sells them, if not then i have to make my own...right? all i have to do is make them to fit the groove side of the board....right?

Place two boards groove-to-groove. Measure that open space and cut your splines to fit snug.

Do you glue the Spline in the Grooves?

I use yellow wood glue and also fasten with whatever fastener I am using on that particular job. A lot of guys say gluing is not necessary.

If the spline is pre-made to match the groove, there is no good reason to glue it. If you made it yourself, you probably should.