I'd like to expand on Aaron's answer that the symbol is a breath mark.
Taking the clue that the page is from "Lutheran church music for the organ", the following is a tip for the organist to accompany hymn singing.
The first system is a suggested fancy organ hymn introduction:
- It starts with the first 7 notes of the first line (to give the congregation a clue) except the next 3 notes ends the first line in a different place with modulation to F major (which grabs the congregation's attention).
- Since metrically the intro finishes the modified first line, there is a breath mark. To execute this, the organist should play the notes of the 2nd beat of the 3rd measure as an 8th note (thus inserting an 8th note rest), but should keep the tempo (no slowdown).
- The final 7 notes modulates back to the key of the hymn (D major) and the top note ends in F#, the same first note for the congregation to sing.
- Although there is no breath mark at the end of the introduction, it is implied. To execute the breath mark, the organist should play the whole note as a dotted half note (3 beats) and a full quarter rest. This is to signal the congregation to start singing after the rest is over.
The next 2 systems is the hymn proper for the congregation to sing, accompanied by the organ:
- For each breath mark, the organist needs to convert the whole note to a dotted half note plus a quarter rest.
- Again, there is also an implied breath mark at the repeat sign, so the whole note also needs to be played as a dotted half note plus a quarter rest.
In this way, the organ breathes along with the congregation throughout the hymn by playing a full quarter rest each time. The result is a beautifully sounded hymn for human voice and organ in a perfect synch because they breathe together.
P.S. Except at the very end of the intro and the very end of the hymn, the pedal should not participate in the rest (thus held for the full note value), thus we have a nice flowing bass line while the other notes participate in the congregation breathing. If not using pedal, the lowest note of the chord needs to be held.