Venetian polychoral style is a fancy way of singing music that comes from a long time ago in Venice, Italy. When people sing music in this style, they use many different groups of singers that all sing different parts of the song at the same time. It's like having different people singing different songs, but they all fit together really nicely.
Imagine you and your friends are all singing at once, but each of you sings a different part of the same song. Some friends sing a high part, some sing a low part, and others sing in the middle. This is what Venetian polychoral music is like, except there are usually more than just one or two groups of singers.
Back when this type of music was popular, people had to use a big church or cathedral because the music was so grand and the singers needed to be spread out across the space. The music itself is very beautiful and emotional, almost like a big hug in music form.
Nowadays, you may hear Venetian polychoral music performed by choirs that specialize in this style, but it still feels just as special and magical as it did centuries ago.