Could someone pls explain what a sandbox is?

When you get your website in Google, so the pages are indexed, but it does not rank for anything apart from the most uncompetitive search terms.

Ah i c - so it means my sites have been in a sandbox for a while, so what causes this?

There's been no real consensus on what's actually involved in the Sandbox. Rand Fishkin goes over some of the mooted theories and ideas here:
http://www.socengine.com/seo/guide/sandbox-march-filter.html

There's a few good reference links in there - hopefully will be useful if you want to explore the topic further.

There is a big difference between "the sandbox" and "ranking poorly" - most sites I have looked at that claim to be sandboxed, just rank poorly.

IMHO, a sandboxed site is:
1) New domain - probably since March or so of this year
2) On page SEO is OK, but not overdone
3) Good number of links, esp with keyword(s) in anchor text
4) Googlebot crawls often
5) Most/many pages are indexed
6) Ranks well for obscure terms
7) Ranks very well for the allins for main keyword (important criteria)
8) Does not rank in top 700 or so for term that its optimized for.
9) Absence of anything that could attract a penalty.

I was never a huge fan of sandbox until recently - have a 3 month old site, that all my experience tells should rank easy in top 3-4 for its main keyword - not even in top 700 and meets all the criteria above.

Additionally:

10) Pages linking to target site, inexplicably rank higher than the target site for the target anchor text.

:)