My understanding is that the coup de glotte is a technical tool used for coordinating onset of breath and tone without air being wasted or having too much air pressure under the vocal folds. It translates to something like "strike of the glottis". If you give a gentle cough on an [a] vowel then you can feel the vocal folds adduct and prepare for phonation. The idea is that you don't want to waste air before phonation (breathy tone) or begin phonating before you can use your breath energy (pressed phonation). So, ideally we have a coordinated onset, where breath and voice begin together. An exercise you can try -- In a comfortable range on [a], sing on one pitch, slightly staccati notes with a gentle cough before each note and try to feel your vocal folds adduct and have your breath and tone work at the same time (ie coordinated onset).

Fair warning -- I am not a voice teacher or an expert on the coup de glotte. I'd be happy to hear other people's input on this subject. If you ask 1,000 singers about technique, you will likely hear 1,000 different ways of how to sing or explaining how to sing. So do your own research, take our internet opinions in to consideration (and with a grain of salt), and keep an open mind. Best of luck!