The man page for /proc (yes, there is one) explains what all those fields mean. In this case, each of those mappings corresponds to one of the sections in the

The man page for /proc (yes, there is one) explains what all those fields mean. In this case, each of those mappings corresponds to one of the sections in the binary. The permissions indicate that the first one is executable code, the second is read-only data, and the third is writable (copy-on-write). The number after the permissions is an offset into the underlying file. I suspect the article is glossing over some details e.g. how gdb figures out which mapping corresponds to which section, but it gets the basic idea across.