Thank you for your query.
The tablet contains a vitamin called folic acid which is essential for the development of a baby's neural tract. It needs to be in your system in adequate amount when you conceive as this is the first system of the baby to develop. It usually needs you to be taking the tablet for three months to get an adequate level for when you conceive. It needs to be continued till at least 12 weeks gestation and there are benefits to taking it till the end of pregnancy too. Vitamin B6 is the other vitamin which has a stabilising effect on your vomiting centre so helps when you conceive. Both are water soluble vitamins and have no long term side effects.
You can read about folic acid and prevention of neural tube defects on www.patient.co.uk
hope this helps