Historically of course it is a finite unit composed of a group of feet. However, the actual definition of foot is very various. The Romans and Greeks used a highly sophisticated patterning by long and short syllables i.e. time duration. Thus, to take a very simple example of what quantity sounded like, here are two lines from Hesiod, written in the six foot line that the ancients called the Hexameter Take notice when you hear the voice of the crane Phracksesthai d', eut'an geranou phonen epakouseis highup from the clouds making its annual shriek upsothen ek nepheon eniausia kekleeguees. One idea which is hard to cope with at first is that a syllable is short or long, as much by what comes after it, as by its own time value.
Long vowels are always long.
In simple terms two consonants before a vowel, whether in the same word or not create a long. The value of this to a modern poet is not in recreating the classic meters, which rarely seem to work in English, but as a useful guide to the patterning of a free verse line. Often to scan a line into its syllable lengths is I find very helpful, in some way it seems to clarify what is wrong. This is especially so, given the tendency of modern English to become much more like American, and much flatter in tone, i.e. with much less accentuation of syllables. Short, short, long, seems common to my ear in modern speech, but there are others.