Google Dance is the name given to the period on which robots are crawling websites means when Google collect all data from their servers, Your feel fluctuation in your ranking. Suppose your website come on first page and that time, when Google collect their data from their server(which you country use) and you check your ranking and you get high drop in ranking or high improvement in ranking, it's called Google dance.
It is not permanent, Your ranking will come on same place and if you work good continuously, you will get better results.