Line lock (or roll control) is an electrically controlled solenoid that goes in the front brake line. The solenoid is controlled with a (Normally off) pushbutton switch that is mounted where it can be easily accessed by the driver, like on the shifter or the steering wheel. Once installed and the brakes lines are bled, you depress the brake peddle first then the switch to activate the solenoid. Then let up on the brakes and the front brakes stay locked until you release the button. This set up is for rear wheel drive. I've never done it, but you could reverse the installation for front wheel drive.
The idea is to lock up the non-drive wheels so you can do a burnout without smokin' the brakes. Also, keeps the car from rollin' when you stage at the lights.