What are metrics in poetry?

Metrics refers to the rhythm of a poem. By rhythm, we mean how the words are said in natural speech.
In spoken language, words are made up of syllables that are either stressed or unstressed, by which we mean parts of the word which are either stronger and more forceful (stressed), or lighter and less laboured (unstressed). For example, 'coffee' is a stressed syllable followed by an unstressed one: 'COFF-ee'. Try saying the word alound. Can you hear it? 'Hello' is another example of this.
Words like 'hedgehog' and 'pancake' have two stressed syllables in a row. Etc.
Can you think of examples?
Metrics, then, is a fancy way of talking about the combinations of stressed and unstressed syllables within a poem.
The broader term for this is prosody, which refers to other structural elements as well as metrics, such as rhyme.
There are words to describe the groupings of syllable stresses. Would you like to learn some today?