In Thailand, the head is considered to be the most sacred part of the body (NEVER touch a Thai person on their head), and the feet are considered to be the dirtiest.
Why does this matter to you?
As a traveller there are a few things you need to be conscious of so that you don't end up insulting the locals.
First of all, traditionally, people sit, sleep, eat and entertain either on the ground or on cushions laid on the ground. As a result, Thai people make sure to remove their shoes (don't want to get the floor dirty!) before sitting in peoples homes or on raised platforms in restaurants or at the beach. Shoes are removed before walking into temple buildings as a sign of respect.
It is also very important to never point or gesture at someone with your foot. This can be taken as an insult.
On the same token, you must never point your toes towards a Buddha statue or symbol. That means that if you are sitting down in a temple, you have to sit with both legs to one side (feet up and pointing backwards) or on your knees. Don't sit with your legs crossed.
Also keep in mind that while chilling out at a beach lounge, in your hotel lobby, or on a train to be careful about propping your feet on the chair beside you. This can be seen as a sign of disrespect, especially if you leave your shoes on.