Just a guess, but it's probably the composer/editor's "helpful" instruction to sing an eighth note and breathe instead of singing a full quarter.
But don't use it...the apostrophe/comma above the barline tells the singers all they need to know. The note in parentheses is an added distraction.
The standard way of indicating special performance instructions that are difficult to notate in standard notation (which this is NOT) would be to use a footnote ( a small asterisk or a number like in a book ) and at the bottom of the page, write: "Sing an 1/8th instead of a 1/4 note, then take a breath and proceed." But you do not need anything like this for this example.
LKent