What is "tailers"

Usage examples of "tailers".

The watchers, that elite team of MI-5 agents who—on their own turf, at least—are reckoned by all the Western services to be the best tailers in the world, were put on alert to envelop Anthony Milton-Rice the following morn­ing as he approached the Defense Ministry with an invisible surveillance for twenty-four hours in every day.

Nor had there been any word from any of the tailers, but I was soon to know where Saul Panzer was.