Interleaving is part of ADSL, a protocol used to connect to the internet over phone lines. This can also be called DSL for those of you in the US.
Interleaving is a technology that increases the stability of the line, at the expensive of latency. Simply put, while it is activated your line will not disconnect as often but will have a slightly higher ping. For most people this is fine, however Gamers and users of a VOIP service or anything where latency is crucial want their ping as low as possible.
Most ISPs turn on interleaving by default, if your line is relatively stable then you can request them to turn it off. Some ISPs will not comply, but most will happily do so. My new ISP ADSL24 (who are a reseller for Entanet) allow you to control interleaving on the line. I requested that it was turned off, and my ping decreased by 10ms. Game on!
Not only this, but my line is just as stable. If you are a gamer or VOIP user, ask your ISP to turn off Interleaving to improve your latency.