Thai culture is hierarchical steeped in respect. An important aspect of this is the wai.

Showing greeting, farewell or acknowledgment, the wai comes in many forms depending on the relative status of those involved, but is usually a prayer like gesture with the fingertips underneath the chin accompanied with a slight bow of the head.

Generally speaking the lower status person will wai first, so for example the young will wai an older person first, an employee will wai his boss, a business man will wai a monk etc.