Best answer I can give you is that a sidechain is designed to run parallel to the main Blockchain and may exchange data with the main Blockchain periodically. In a lot of cases, the sidechain can send data or receive data from the main Blockchain, but can't do both. If you've ever heard of Bitcoins being "burned," that means those Bitcoins are being locked up in an unspendable form by transferring them to a "one-way" sidechain that can't transfer them back. Sidechains are often used for experimental purposes -- for instance, you might transfer a few Satoshis from the main Bitcoin Blockchain to a sidechain to test a new feature you're working on.
Thanks for your answer commandrix!

Does the creation of a sidechain requires creating a new crypto-currency ? Or can you interact between the main blockchain and the sidechain with the same crypto-currency ?