how does below tr command replace nonletters with newlines?

I think I understand tr -cs '\n' part.. but what is

A-Za-z\' <--- what is this??

tr -cs A-Za-z\' '\n' |


-c --complement

-s, --squeeze-repeats
replace each input sequence of a repeated character that is listed in SET1 with a single occurrence of that charac-
ter