Can someone explain what is Meter?

I have always wonder what do poets mean when they say the poem is meter..?

How does that work?

Poem is music! This part is restricted with same syllables or rhythm on each line, and there are many forms we heard have each meter. Each form has their own meter. When we write we able to recognize the fluency of poem, it's easier to edit without racking the brain because we accustomed to write the form based on learning. Sonnets have many syllables and rhyming poem has lesser than sonnets. Free verse does have, but third line never rhymes with fourth line and only first and second does internally.
Each form has different rhythm. :)