Tone refers to a change in the pitch of a syllable during its pronunciation. In Thai, every syllable is pronounced in one of five tones: low, mid, high, falling, or rising. The tone must be spoken correctly for the intended meaning of a word to be understood. Since every word has a particular mandatory tone, we say that the Thai language has obligatory lexical tones.
The middle tone starts at a middle pitch level, rises slightly and returns to mid-level. The low tone starts low and gradually falls even lower. The falling tone starts high and falls to a low pitch. The high tone rises to a peak and then drops. The rising tone starts at mid-level and gradually rises.
Modifying the vowel length or the tone changes the syllable completely, to the extent that if you mispronounce a single vowel or tone, a Thai person hears a different word to the one you meant to say.
Tones are realized in the vowels but indicated in the script by a combination of the class of the initial consonant (high, mid or low), vowel length (long or short), closing consonant (plosive or sonorant) and, if present, one of four tone marks, whose name derive from the name of the digits 1–4 borrowed from Pali or Sanskrit.
As an example of how the tones can make things difficult for learners of Thai, here is a sentence that means "new wood doesn't burn, does it?" In Thai to western ears, it sounds like "mai mai mai mai mai", just with varying the tone on the syllables. Of course, to Thai ears, it is understood just fine.
Of course, learning the five tones can be difficult at first. But once you have broken the tone barrier, you have grasped the core of the Thai language. Regular practice, practice, patience is essential in mastering Thai tones.