This is a potentially vast subject
So you'll be pleased to know I'm taking the minimalist approach. Perhaps not quite the minimalist approach because then I'd just say that metre is nothing more than the rhythmic sound within a line of verse (or indeed a line of prose or speech).In poetry, the metrical systems used are often more or less regular. There are four such systems, quantitative, syllabic, accentual, and accentual-syllabic. The last is by far the most common in modern, or even reasonably modern, English verse.
It can often often be more helpful to think instead in terms of feet, the group of syllables that are normally taken as the standard unit in poetry. The line known as the dimeter has two feet; the trimeter three; the tetrameter four; the pentameter five; a hexameter (also known as the alexandrine, and common in French verse) six; and a heptameter seven. The stressed syllables in the line are the most 'important', and the number of stressed syllables is often taken as the 'trademark' of the line: the iambic pentameter (five stressed syllables each followed by five unstressed syllables) is the most common metre found in English verse. It is common enough in free verse, and even ordinary speech. The usual way to record metre is to show stressed syllables as a forward stroke (/), and an unstressed syllable as a cross (x). A 'normal' line in iambic pentameter would thus be shown as / x / x / x / x / x. It is usual to vary the length of line a little, especially in longer poems, to help avoid the pitfall of doggerel.