One female mason bee typically fills several 'nests' (=reeds, in this case) during her lifetime. Solitary bees typically live four to six weeks. 

Inside each completed nest (below, plugged with green leaf material), is a linear series of sealed chambers, each one containing an egg atop a ball of pollen. Over the next few weeks each egg will consume the pollen ball and grow into a worm-like thing (a "larva"), then eventually an adult bee  by late summer. Growth then halts --bees hibernate-- until the following spring. When temperatures heat up again, bees are ready to go! They emerge by chewing their way out of the sticks, usually in sequence (front --> back). Amazing!

Check out the progress of 2 weeks:

Why they prefer the used reeds (on the outside of the box)... I can't explain.