A
grave accent (pronounced
grahv, not like the hole where you put someone) is the small
backslash effect that is so familiar in western European
languages. The HTML symbols are à etc.
Except that it's not that common. In French it's used on e, as in mère, and when so used it indicates a mid-open (= mid-low) vowel, SAMPA symbol [E]. (In contemporary French the spoken distinction between è and é is often absent.) It's also used on a in three words I can think of, à, là, déjà. If there are any others they don't materially affect its scarcity.
In English, apart from straight borrowings from French, it's preserved in a couple of names, such as St Thomas à Beckett and in Thomas à Kempis (author of the Imitation of Christ), where it's not the French preposition, but the English at = 'of, from, de, von' (seen as atte in the odd place name).
In Italian it indicates stress at the end of a word, and is used on all five vowels: sarà 'will be', perchè 'because', così 'thus', Salò Republic, and virtù. I believe there is a recent move, led by the printing house Giulio Einaudi, to use the acute over close and mid-close vowels, as virtú, but I haven't seen much modern Italian text, so can't confirm this.
In Catalan it indicates stress. In Portuguese it also does in odd circumstances: as in the adverb fàcilmente from fácil 'easy'. It also makes the word à, which is a contraction of a a 'to the (fem.)'.
In Scottish Gaelic it indicates a long vowel. In the 1970s a spelling reform scrapped the acute accent; before that they were used distinctly, the grave for mid-low vowels (SAMPA [E] [O]) and the acute for mid-high.
That's about it as far as western European languages. It's never used on consonants. It never forms a distinct letter of the alphabet.
It's not much used in modern Greek, which these days just uses the acute for stress. The origin of the grave accent is in Ancient Greek: it was invented, like the other accents, by the grammarian Aristophanes of Byzantium, who was concerned at the way the language was changing, and wanted to mark for learners how it should traditionally be pronounced.
Greek was shifting from a pitch accent to a stress accent. As a tone language, Greek had two pitches, high and low, and a long vowel could also have a tone falling from high to low. Aristophanes (not the playwright) used the acute to mark high, the grave to mark low, and the circumflex, the combination of the two, for the fall.
As a rule only one syllable could have a high tone, and all the others were low, but they weren't all marked with a grave: only the one that might be high, or looked as if it might, under some circumstances.
In twentieth-century linguistics the grave accent was used as a symbol of falling tone, which it visually reminds us of, and was officially so used in the IPA, in the pinyin system for Chinese, and also in the official orthography of a number of African languages. However, fairly recently the IPA scrapped its whole system of pitch marking and adopted a new one, in which the grave accent was once more given its original meaning of plain low tone.