Thiamin (or Thiamine) is also known as vitamin B1, which is makes up one of the many water soluble B Vitamins. Like Riboflavin (vitamin B2), thiamin is naturally present in some foods and can added to other foods and dietary Supplements like the one's you see below.
Vitamin B1 aids in the production of energy & the normal function of nerve cells. If thiamin deficiency occurs, the body's neurons may become damaged or even die.
Foods with a decent source of thiamine include Nuts & Seeds, Oats, pork, beef, liver, legumes, eggs and yeast. Food that have vitamin B1 added to them are known as 'fortified foods' and include Flour, Bread, Pasta and breakfast cereals.
As thiamin is a water soluble vitamin, excessive levels are extremely rare because the body will simply excrete them through urine.
As different people respond differently to supplementation, it may take at least 2-3 weeks for thiamine to start working, depending on what you're taking it for. It is widely regarded that those who supplement with thiamin for longer reap its benefits.