In wrestling, a stranglehold is a strong grip around an opponent's neck. Figuratively, you have a stranglehold on something (or someone) when you have complete control over it.
The powerful hold that's also known as a chokehold is used in various martial arts, as well as police and military training. A true stranglehold does just what its name describes, constricting a person's breathing. Even if someone doesn't literally have you by the neck, you might feel a stranglehold being exerted by things like stress, deadlines, or debt.