You can decipher the meaning of totalitarian by the first part: "total." It refers to a government with total power, one that exercises complete, even oppressive control over the people and their activities. (You can also figure it’s not nice.)
The word totalitarian first came about in 1926 as totalitario, an adjective to describe the Italian fascism of that time. The English form was adapted from the Italian to describe an absolutely powerful regime. Socialist leader Norman Thomas once said: "To us Americans much has been given; of us much is required. With all our faults and mistakes, it is our strength in support of the freedom our forefathers loved which has saved mankind from subjection to totalitarian power."