For the basic information of the bandhas, read this article: bandhas.
In Sanskrit, Jalan means net and Dhara means stream, flow.
Bandha is a Sanskrit word that means to hold, tighten or lock.
The word Jalandhara has various interpretations.
The most likely is that it means “net” or “cluster of nadis” or “energy pathways”.
Therefore, jalandhara is the practice that controls and regulates the energy in the network of the nadis in the neck and throat regions.
Jalandhar bandha is used to stop the flow of nectar that is falling from the bindu chakra to preserve/save it in the throat.
Hatha Yoga Pradipika is mentioning: "Nectar oozing from the moon is restricted above the neck region and thereby it can't be swallowed up by the sun at the navel."
Another one more explanation we can find in some of the Hatha texts is that there are 16 adharas (specific energy centers) in the body. Jalandhara bandha's aim to bind the energy of these all adharas through contracting the throat muscles.
According to Gheranda Samhita 3:10-11
Contract the throat and place the chin on the chest. This becomes jalandhara bhanda. It gives control over the sixteen bases. This great mudra by the name of jalandhara bandha gives victory over death, mastery over jalandhara bandha provides siddhis to yogis and by practicing it for just six months a yogi definitely becomes a perfected being.
Sit in any comfortable meditative asana.
Make your knees firmly grounded.
Place the palms on your knees.
Relax your whole body.
Inhale deeply. Retain the breath inside the lungs.
Bend the head forward and press the chin slightly against the chest/sternum.
Keep your arms straight, elbows facing each others.
Your hands should firmly remain on the knees, this action will tend to intensify the pressure in the region of neck.
Hold according to your capacity.
Then, slowly release the chin lock and exhale deeper and longer through the nostrils.
This is one round. After normalizing the breath, repeat the same thing.
Beginners should start their practice with 5-7 rounds and later on slowly increase the number.
On the region of the throat.
Practitioner will never become old (according to shastras).
Controls the flow of prana in the body.
Helps to relax the mind.
Very good practice for proper functioning of thyroid and parathyroid gland.
Helps to regulate the metabolism.
The throat is also considered the place of vishuddhi chakra so this bandha will help to awaken this chakra (communication).
Develops meditative introversion and one-pointedness.
In kundalini yoga, the practice of Jalandhar bandha is used to change the speed of prana and to awaken the vishuddhi, ajna, bindu and sahasrara chakras.
People suffering from high blood pressure, heart disease, epilepsy, vertigo should not practice or ask your teacher.
Whenever you practice any kumbakha or bandha, never force yourself.
During the practice, if anybody experience discomfort, you should immediately stop the practice and relax yourself.
If you are going to practice for first time, practice in the guidance of a teacher.