In famous "Turot eszik a cigany" (aka "See the Gypsies Munching Cheese") by Kodaly Zoltan I've encountered with these grace notes, marked on the picture:
What are they for? How to sing them? Maybe like a glissando? I barely think so.
I haven't heard any performance with those neither on YouTube, nor at our concertos (we performed the arrangement for mixed choir).
Those notes are called "Acciaccatura" or sometimes called "Short Appioggiatura" Those notes are to be sung as fast as possible. Those are nonchordal notes which mean the emphasis should remain on the chordal note.
You have some artistic licence in regards to how you approach the beat. You can either play the grace note as part of the beat, or you can let it fall in with the last part of the previous beat, but either way the grace note should be as short as possible, and it should not take the beat away from the chordal note.
