It means to rock a baby to sleep, and just about the only time it's used these days is in the lyrics of a nursery rhyme called Rock-a-bye Baby. So, yes, it's relatively rarely used. The word "lull" means to soothe. You can lull a baby to sleep by singing to it and/or by rocking its cradle. Another term for sleep is "bye-byes" and over time "lull to bye-byes" turned into "lull 'a bye" and then lullaby. If you lull by means of rocking then you have rockabye or rock-a-bye.