All councils have a certain amount of surplus cash esp as it is only half way through the finanical year. This extra cash is invested and the interest earned is usually re-invested (the investment grows year by year earning more interest).
Take Kent cc for example they invested �40M which if you assume an average coucil tax of �1200 per household is only 33,333 households which is a tiny percentage of the overall income generated by all the households in Kent.
Like most people they put it there as the Icelandic banks were offering the most competitive interest rates on the market, much more than British institutions. Icesave for example was one of the accounts Martin Lewis from Money saving expert recommended.
What this all could mean is explained in detail here.
http://news.bbc.co.uk/1/hi/business/7625419.st m
A jargon buster is here it helps explain some of the wors and phrases used in the press and on tv.
http://news.bbc.co.uk/1/hi/magazine/7642138.st m