what is deffered revenue?

Deferred revenue is income that is not taxed until some time in the future, i.e., "deferred". An example of deferred income is the money placed in a traditional IRA. It is not taxed until it is withdrawn, hence, "deferred". This is true of all "qualified" retirement plans, such as 401k plans, variable annuities, Keoghs, etc.. Also, sometimes a corporation will "defer" income so as to receive it in a more favorable tax year.

Deferred Revenue
A liability account used to collect deposits and other cash receipts prior to the completion of the sale.

Notes:
Deferred revenue is important because it's the money a company collects before it actually delivers a product. For example, a software company sells and receives payment for a computer program before it gets delivered or installed. This doesn't get recorded as straight revenue because, if something goes wrong with the job, the money is at risk.