A while back means some time in the past.Also, a while ago. This term uses a while in the sense of "a short or moderate time," a usage dating from about 1300.

Note:

A while back意思是一段时间以前，过去的某个时间。也可以说a while ago。从1300年开始，while可以 代表并不长的一段时间。

例：I ran into Barbara a while back but didn't get her new address.
        我前阵子偶然遇到Barbara，但是没得到她的新地址。