This common English idiom means to be in charge. It specifically implies:
1. Being in charge of one's own house
Though its often used to refer to
2. A person, group or team just being really really good at what they do.
1. "Despite appearences, Jenny really rules the roost".
2. 'Arsenal have really ruled the roost this season".
One theory places this expression as far back back as the 15th Century. The sense, or course, is that of the rooster ruling over the chickens in the roost, or chicken coop.
But bizzarely enough, there is some suggestion that the phrase is actually much later, and a corruption of "ruling the roast", referring to the person who would carve up a roast meal, and hence the master of the house. Webster backs this one up -- check out under roost.
The writer W C Hazlitt mentioned this saying in his book "English Proverbs and Proverbial Phrases" .