Glottis refers to the opening between the vocal folds as you can see in the image below. If the vocal folds are apart then the glottis is open. If the vocal folds are pressed together then the glottis is closed.
Note: The images below show the inside of the larynx (which is in the neck) from above.
The changes produced by vocal folds are often important in speech. There are four usual positions of the vocal folds that influence our speech.
For normal breathing the vocal folds are wide apart as is shown in the image below. Usually, producing voiceless consonants like p, f, and s requires vocal folds to be wide apart.
The fricative sound h requires a narrow glottis. The h sound produced this way is quite similar with a whispered vowel. This sound is called "voiceless glottal fricative".
The vibration of the vocal folds is caused when they are touching (or nearly touching). The air that passes through the glottis causes this vibration. It comes from the lungs and it is under pressure. This pushes the vocal folds apart and allows a small quantity of air to escape.
The air escapes fast through the glottis causing the vocal folds to come close together again. This open-close of vocal folds happens quickly and regularly causing the vibration. It is said that this vibration happens about 300 times in a woman voice and just 150 times in a man voice on average.
The vocal folds can be completely closed so the air can not pass through. In speech this is called a glottal stop (glottal plosive as I like to call it). The symbol use for a glottal plosive looks similar with a question mark symbol “?”. The symbol is ʔ.