>> what is sidechaining and how to use it?

[Timo]

Yup, slapping an EQ unit into the side-chain lets you control which frequencies will dominate the compressor action. Placing a graphic eq into the side-chain and boosting the high freq bands, for example, will make the compressor duck more when HFs are present = resulting in a "de-hisser" type action.
Likewise, attenuating the lower freq bands will stop the low end dominating the compressor action (seeing as LFs are the most powerful), resulting in less of a severe compression effect when heavy LFs are present. (Like kick drums, or bass lines, which would otherwise kill the rest of the audio spectrum as the compressor reacts to the most powerful signals - the LFs - more than any others, especially when a single-band compressor is being used).

This next link is a MUST READ. It discusses the more artistic side of compressor (and especially side-chain) usage, turning an otherwise single-character compressor into many others:

The Compressor's Secrets (and in PDF format which includes the illustrations).

An absolutely fantastic article.