Can someone explain in laymans terms? My ISP allows me to disable it on my ADSLMax line.

Why would I want to? Are there any downsides in doing so?

Thanks.

In my experience, unless your line is extremely stable and with practically no noise then leave interleaving turned on.

Interleaving is a way of 'chopping up' the data so that any interference in the connection has less of an impact. You can check the details here: http://www.kitz.co.uk/adsl/interleaving.htm

With interleaving off, you should get faster speeds, but at the expense of reliability.

I agree with everything the above person wrote, but also want to add that with interleaving turned on it'll add some extra latency to your line.

If you play any online games where you want/need the lowest ping possible, then you probably want interleaving off.