For the basic information of the bandhas, read this article: bandhas.
In Sanskrit, Moola means root, origine one or the first one, source or cause. It refers to the root of the spine or the perineum where mooladhara chakra, the seat of kundalini or primal energy, is located.
Bandha means to hold, tighten or lock.
This practice involves a physical lock applied on the perineum, between the anus and the genital organs.
Very beginners may feel little bit the contraction on the lower part of the belly also.
In the male body, the area of contraction is between the anus and the testes.
In the female body, the point of contraction is behind the cervix, where the uterus projects into the vagina.
According to Hatha Yoga Pradipika 3:61
Pressing the perineum from the part of the heel of the root, contract the anus. Raising the apana vayu upward (in this way) is named mulabandha.
Sit in any comfortable meditative asana, preferably siddhasana or padmasana.
Make your knees firmly grounded.
Place the palms on your knees.
Relax your whole body.
Inhale deeply through the nostrils, then hold the breath by the chin lock (antar kumbhaka).
Contract the muscles or anus sphincter muscles, pull it upward as comfortably as you can.
Hold this contraction as long as possible.
Then, release the contraction, relax the anus sphincter muscles, and slowly release the jalandhar bandha (chin lock).
Exhale deeply and longer.
Rest here for 3-5 natural breaths. This is one round. Then, repeat the same thing.
Beginners should start their practice with 5-7 rounds and can increase the practice gradually.
On the place of mooladhara, or on the point of contraction.
According to Hatha Yoga Pradipika 3:65
As a result of regular practice of mulabandha, the apana and prana vayu get united, urine and faecal matter also reduce and even old becomes (like) young.
We practice this bandha with jalandhar bandha so that all the benefits of jalandhar bandha are obtained.
Very good practice for the proper circulation of the blood, specially in the pelvic region.
Good for the purification.
Helps to revitalize all the organs of the pelvic region (colon, bowels and blader, genitals and anus sphincter muscles).
Very good practice for those who are facing the problem of constipation, peles, fistula...
Beneficial for women whose periods are not regular.
Calms the mind by releasing the negative emotions.
Moola bandha is a means to attain sexual control. It may be used to sublimate sexual energy for spiritual development (brahmacharya), or for enhancement of marital relations.
Is effective for awakening mooladhara chakra.
Don't practice during menstruation.
Stomach, bowels and bledder must be empty.
In the beginning, practice ashwini mudras in preparation for moola bandha.
Moola bandha raises the energy, so practice in the guidance of a teacher.