1Z0-874 Web TestEngine demo

Consider the following:

mysql> EXPLAIN SELECT * FROM City WHERE CountryCode = 'USA'\G *************************** 1. row
***************************

id: 1

select_type: SIMPLE

table: City type: ALL

possible_keys: NULL

key: NULL

key_len: NULL

ref: NULL

rows: 4079

Extra: Using where

What does the possible_keys column in this output denote?