Tightlining is a method of applying eyeliner to the base of the lashes, in order to give very natural definition and the illusion of a fuller lashline. You add a couple of drops of water to the cake, then mix with the brush to give a slightly thicker than ink paste. This is then applied to the lashline by wiggling the brush into the lashline from underneath - it's not applied to the waterline but to the very base of the lashes.
Here's my bare eye (with concealer and brow pencil.)
Here's the same eye with the Bleu Marine liner applied by tightlining. I love the Bleu Marine - it's slightly softer than black, and very brightening and flattering, but it doesn't look blue when it's applied.
Honestly, it does take some time to get the hang of the technique; it's well worth getting a lesson at the counter if you're considering buying this, and make sure you have plenty of cotton buds around when you try it at home. I apply mine before I apply eyeshadow, so I can correct any smudges with a damp cotton bud before applying eyeshadow. I love the look of it though, and for me it was worth the effort of learning the technique. You can also use the Laura Mercier eyeliner to do a traditional line on the top of the lashes too, for a more dramatic look.