Infiltrators or collaborators with the enemy.
In October 1936, in the hostilities of the Spanish Civil War, the nationalist General Emilio Mola and his supporters besieged Madrid with four columns of troops. Mola claimed he had additional troops within the city. The claim was reported in the New York Times like this:
Police last night began a house-to-house search for Rebels in Madrid... Orders for these raids ... apparently were instigated by a recent broadcast over the Rebel radio station by General Emilio Mola. He stated he was counting on four columns of troops outside Madrid and another column of persons hiding within the city who would join the invaders as soon as they entered the capital."
Explicit mention of the hidden troops as the 'fifth column' was reported in the Fitchburg Sentinel newspaper, on 14th October 1936:
"Out of hiding came a few of the phantom 'fifth column' - the fascist auxiliary force dreaded by the loyalists."
The term has migrated in use over time and is now sometimes used more generally, to mean traitor or spy.
Ernest Hemingway wrote a play called 'The Fifth Column' in 1937, in which he expressed his opposition to the Spanish fascist regime.