A figurative device, in which a woman hold's a man's manhood, preventing him from acting as per his natural state. Implicit in this arrangement is that the woman has means of reprisal against the man, and should he misbehave, grave consequences await. Most commonly used to describe arrangements in spousal or significant-other relationships, but can also be used in other scenarios where a woman has power over a man, and he must accede to her demands or face consequences. Often abbreviated, just 'TLB', especially when used in polite company.
The word is only correctly used if the man feels emasculated as a result - it does not cover all situations where a woman has authority over a man. A female boss who just threatens to fire a guy would not have him in a testicle lockbox if he feels machismo because of it.
In the late 90's, perhaps the genesis of the term, it was used to describe the arrangement Hillary Clinton had come to with Bill Clinton after the Monica Lewinsky scandal erupted.
Q: Joe, you wanna go bowling tonight?
A: Naw, man, my old lady, she don't want me being out late.
Q: She's totally got you in a testicle lockbox.