How Do I Transfer My Backups on My Main hard drive to my external hard drive?

I only have 31 GB of storage on my Macbook Air. Just a week or so ago, I had about 55 GB. When I checked what was eating up my memory, it said 30 GB of the memory was used for backups. I figured if I plugged in my external hard drive and ran a backup on it, it would transfer those backps to my external hard drive, but it didn't. My question is, how do I transfer that 30GB of backups onto my external hard drive so that I can free up some memory on my main drive?