A bar. as the word is used in Tennyson's poem, is the place where a river flows into the ocean. Crossing a bar in either direction is a turbulent experience, especially if it is a big river and a small boat. The strong river current meets with the much greater power of the sea, causing the boat to bob up and down at the same time it is listing from side to side. The boat in Tennyson's metaphor is crossing the bar in the direction of the sea, like going out into infinity. The river represents life. "Sunset and evening star" suggests that life is ending. The bar represents the end of life and the frightening experience of passage from one familiar and comfortable state into another which Hamlet calls "That undiscovered country from whose borne no traveler returns." The sea represents the peace and stillness of eternity. The pilot is obviously God, and Tennyson is suggesting that God has been directing him all his life without his yet having seen this omnipotent guide face to face. The poet is expressing his religious faith that death may or may not be the end of everything but that at least the mystery of existence may be revealed to him in his last moments. 

According to the eNotes Study Guide:

The enduring popularity of this short, meditative lyric lies in its ability to appeal to many people on many levels, despite attempts to limit it to one interpretation. 

Although the poem consists of a complex metaphor, most readers immediately understand and relate to its implications. Everyone has to die sooner or later, everyone's life experience is like being borne along by a river, and everyone has to cross that frightening bar. Tennyson sounds like a typical Victorian. He doesn't say he expects to meet his pilot face to face--but only that he hopes to meet Him face to face.

A less hopeful view of death is expressed by Tennyson's contemporary Algernon Charles Swinburne in one of his best-known poems, "The Garden of Proserpine":

From too much love of living,
        From hope and fear set free,
We thank with brief thanksgiving
        Whatever gods may be
That no life lives for ever;
That dead men rise up never;
That even the weariest river
        Winds somewhere safe to sea.

In this poem, it seems pretty clear that the Pilot represents Jesus or, as Tennyson himself put it, "that Divine and Unseen Who is always guiding us."

The poem as a whole is talking about the death of the narrator (which will, apparently) come pretty soon.  Unlike some poems about death, this one does not talk about struggling against death or fearing it or being in any way sad about it.

Instead, the whole tone of the poem is very content.  The narrator is not worried about death because he will (he hopes) meet God (or Jesus) face to face after he dies.