The photo is a place holder at the moment - 6L6GC is the closest equivalent to the 5881. You will receive 5881(s), not the pictured 6L6GC.
5881s are essentially a military spec version of the standard 6L6 (not to be confused with the slightly more powerful 6L6GC).
6L6s are power amp valves commonly used in American styled amps. They're an octal power valve of approximately 25 Watts of output. The 6L6 is a beam tetrode output valve, meaning they have additional plates to focus the electron stream into beams to increase the output of the valve. They function in a different way to a pentode (like EL34s for instance), giving them an different tonal structure.
6L6s are commonly considered to be scooped and contribute to the thick bass end and crystal clear top end found in many early 6L6 equipped amplifiers of the 50s.