“Significance of Quatrain in Literature
From the prevalence of different forms of quatrains in many different literary traditions, it is clear to see that quatrains have been a building block of poetry for a large part of human history. Though it has been so popular for many millennia and in many different cultures, there is no unified theory about why the quatrain is so fundamental to poetry. It could be that its brevity makes the form easier to memorize, which was important in the early days of oral storytelling and the tradition of nomadic storytellers later on, such as troubadours. Examples of quatrains also show that they can be just long enough to get an entire sublime concept across to the reader or audience member, such as in the tradition of Shichigon-zekku”
How my heart sings 
I know little about poetry except the power of some poems to speak to my heart directly. From my vanishing knowledge of Latin I think a quatrain has something to do with four lines. Is it four rhyming lines or just four lines that express a single thought?
If you click on the link it explains it.There are many kinds but basically it is 4 lines with rhymes: AABB or ABAB or ABBA etc.I like them because it’s a simple form and can be very expressive w:)