"Eddie Ate Dynamite, Good Bye, Eddie" is a mnemonic device for what?

"Eddie Ate Dynamite, Good Bye, Eddie" is one of a handful of mnemonics to remember which pitch, from lowest to highest, 6-string guitars are tuned to.

These pitches are for open strings, which are strings that are not fretted. These are Low E (E subscript 2), A, D, G, B, and High E (E subscript 4). On the guitar, low to high is from top to bottom. Tuners are cheap (even free with smart-phone apps), but a lot of guitarists still tune by ear; they'll compare their open Low E with a sample, then fret that string on the 5th fret, compare the open A to the fretted E, and so on up the strings (or down the neck), with the only exception being to use the 4th fret on the G string for comparison to the open B string.