Minecraft wiki entry for NBT_path_format, associated text: foo—Specifies the subtag named "foo" under the root tag.
foo.bar—Specifies foo's child named "bar".
foo.bar[0]—Specifies the first element of the list "bar"
foo.bar[0]."A [crazy name]!"—Specifies the child named "A [crazy name]!" under that first element
foo.bar[0]."A [crazy name]!".baz—Specifies the child named "baz" under that crazily named element