This is about how to differentiate the waxing moon from the waning moon in the sky. »Waxing« is known to be the lunar phase between the new moon and the full moon (meaning the time when the crescent increases in size), whereas »waning« is known to be the lunar phase between the full moon and the new moon (the time when the crescent decreases in size). In order to remember on which side the curve is, here are some neat mnemonics.
A well-known example in English for the waxing moon is to use the spelling of the lower case letter »b«, which has a curve on the right hand side and therefore signifies »beginning«. For the waning moon, the »d« is used which has its curve on the left hand side, representing »descending«.
• ☽ = »b« = beginning (waxing)
• ☾ = »d« = descending (waning)
Another mnemonic describes the order of the waxing crescent → full moon → waning crescent, uses the shape of letters too, in this case the »D« and the »C« and creates the abbreviation »DOC«:
• ☽ = »D« = waxing moon
• O = »O« = full moon
• ☾ = »C« = waning moon
These mentioned mnemonics only work in this way in the northern hemisphere of the Earth. For the southern hemisphere you have to reverse the mnemonics (so to say »to turn them topsy-turvy«), because there, the curve of the moon is just the opposite for the observer.