These couplers have 1/8" Standard NPT threads.  A 'Coupler' usually means that the piece is the same on both ends so Female to Female or Male to Male.  

The F to F would be perfect for connecting two 1/8" NPT Braided hoses together. Or maybe putting a gauge on the end of a hose.

The M to M will often be used in some sort of manifold assembly fitting together various parts.  Maybe attaching 2 ASA's together for a multi bottle setup.

'Male' threads mean the threads are visible on the outside and the part goes into something else.

'Female' threads mean the threads are internal and other parts will thread into the fitting.