"Crossing the Bar" has nothing to do with making eyes at the hottie at the other end of a drinking establishment. The bar here is a sandbar—you know those bars of sand at the mouths of harbors and rivers? They make a boundary of sorts, protecting the harbor from rocky waves, and keeping folks from setting sail on the open sea with too much ease.
To leave the harbor behind and sail out on the open ocean—to cross the bar—is Tennyson's metaphorical way of talking about death. Why that particular metaphor? Well, if you think about it, it actually makes perfect sense. Death is merely the crossing of another type of boundary or barrier (the "bar" between life and death, real life and the afterlife). When somebody dies, they leave the safe harbor of life and enter the vast unknown of the afterlife, whatever that may be.
The great unknown that is death is a lot like the ocean. The ocean is massive, huge, so big that it is sometimes impossible to comprehend. And you know what? So is death. It's a huge part of everyone's life—the end of everyone's journey. Yet somehow, we don't know anything about it. The only difference here is that, when we cross literal sandbars, we can always turn right back around and head back to harbor. But in life, once we cross that bar, there's no going back.