In
phonetics, this can mean one of two things. It might mean a
vowel with a high
pitch, in a
tone language, but usually the
high refers to the
tongue position. In a high vowel the centre of the
tongue is up towards the roof of the mouth. The furthest forward it can go makes the sound
[i] as in
machine, and the furthest back it can go makes
[u] as in
rule. Almost all languages have something close to these two vowels, with the lips
spread for
[i] and
rounded for
[u].
Sounds with the opposite arrangement of lips occur in some languages. When [i] has lip-rounding it is the front rounded vowel, phonetic symbol [y], which occurs as German and Turkish ü, French u, Dutch uu, and many more. Sometimes in English-language phonetics, the symbol [ü] is used instead of [y].
The reverse is less common, a back unrounded vowel, [u] with spread lips. But it occurs in Japanese, in the sound we transcribe as u. In Vietnamese it is Ư, that is a U with a hook (Unicode call it a horn) attached to the right. The IPA phonetic symbol for this is an upside-down [m].
High vowels always have the mouth relatively closed (so a lip-reader goes on lip shape, not tongue position). For this reason high vowels are also commonly called close vowels, and this is the official terminology of the IPA. Because of the ambiguity with high pitch, it might be better if they always were called close. But the term 'high' is well-established, and I feel more comfortable with it, because it directly describes the physical formation of the vowel.
In between the furthest front and back vowels are central vowels, which occur in a number of languages both rounded and unrounded. The IPA phonetic symbols for these are [i] and [u]. The Turkish undotted-I letter ı is unrounded, somewhere between back and central. The Russian letter Ы is central unrounded, and the Polish y is similar. The high central rounded vowel occurs in Scots and some modern southern English pronunciations of good ("guid"). I believe that in Swedish the letter u is a similar vowel.
The high vowels are the highest the centre of the tongue can move while not touching the mouth, i.e. while still being the unobstructed sound known as a vowel. When you move higher than [i] you start to get slight friction with the front of the palate. This is the consonant y in yes, you, phonetic symbol [j] (though [y] may also be used). When you move higher than the back vowel [u] you get the consonant [w]. In fact the physical and phonetic difference between them is usually negligible, and when these consonants are frictionless, which is almost always, they are called semivowels. The real difference between [u] and [w] is that the former occurs in the nucleus of a syllable and the latter at the edge of it.
There is also a (very rarely occurring) semivowel corresponding to the front rounded [y]. The phonetic symbol for this is an upside-down (turned) [h]. The only instance I know of this is in French: the word lui, huit are monosyllables [lYi, Yit] with this semivowel rounded-y.
Diphthongs (vowels that move from one position to another within a single sound) often move towards a high vowel. Many languages have diphthongs [ai] (as in aisle, eye, high, sky) and [au] (as in mouse, cow). These are sometimes phonetically transcribed with the corresponding semivowel symbol, thus [aj, aw] or [ay, aw]. I think this is a national habit of phoneticians (very common in America) rather than a precise phonetic distinction.