The “babel” ssid is used for ad-hoc mesh networking in cerowrt. “babel” is just a convention, any ad-hoc SSID would do. Other users use “mesh” or something generic like that.
Cerowrt’s mesh networking uses an alternative address distribution protocol (AHCP), rather than dhcp, which supports distributing ipv4, ipv6, ntp, and dns server information. It is how cerowrt routers (or any device using ahcp and babel) mesh together, if two are in range of each other, and on the same channel.
While it would be possible to make “babel” be a hidden, rather than announced, SSID, it seems to be best to make it visible and advertise its availability. Note that although a “babel” SSID might be available, you do have to set or autoselect the right wifi channel to use it from a host.