Please realize that Sir Thomas Malory makes sure this quotation from Arthur's grave, yes, his epitaph is written in Latin and not English. Therefore it reads as follows:
Hic jacet Arthurus, Rex quondam, Rexque futurus
Even without knowing Latin, but knowing quite a few English root words (I would hope), you will recognize Arthur's name (Arthurus) as well as the word Rex (King), ... a variation of the word Rex which is Rexque (again, King), as well as the word futurus (which sounds a lot like our word future). Because we get much of our English language from Latin, it's easy to figure out the exact meaning here. Here Lies Arthur, Once King, [and] Future King.
What precedes this burial and epitaph is the Mordred vs. King Arthur battle where Mordred was slain and Arthur was wounded, never to recover. At this point, Arthur asks for the famous sword, Excalibur, to be thrown back into the lake. His request is finally granted, and it is caught by a hand (the Lady of the Lake?) and sucked under the water. It is only then that King Arthur dies and his body is put on a boat and sailed down the river to Avalon.
Further, when the words of the epitaph claim Arthur to be Rexque futurus, that means that the myth of King Arthur continues: specifically the myth that King Arthur will some day rule again and, therefore, is not really dead (and/or will rise like Christ). In this regard, Arthur is said to be England's savior, ... someday. It also hints at the great chivalric code revolving around King Arthur: always a king who held honor dear.
For I have promised to do the battle to the uttermost, by faith of my body, while me lasteth the life, and therefore I had liefer to die with honour than to live with shame ; and if it were possible for me to die an hundred times, I had liefer to die oft than yield me to thee; for though I lack weapon, I shall lack no worship, and if thou slay me weaponless that shall be thy shame.
Malory alludes to the fact that some said that Arthur was not dead and shall come again in Britain's time of need:
"YET some men say in many parts of England that King Arthur is not dead, but had by the will of our Lord Jesu (sic) into another place; and men say that he shall come again, and he shall win the holy cross"
So when Malory tells us that some men claim his tomb says, "Hic jacet Arthurus, Rex quondam, Rexque futurus,"(Here lies Arthur the once and future King) he is referring to the belief that Arthur is not really dead and will come again at some future time for Britain's salvation. The claim also gives Arthur a somewhat godlike quality since the only man who has ever died and expected to come again is Christ. T.H. White took this epitaph as the title to his rendition of the Arthur Legend: "The Once and Future King."