What does the grave accent mark on words mean?

Solution 1:

Wikipedia says it at least as well as I could have:

The grave accent, although not standardly applied to any English words, is sometimes used in poetry and song lyrics to indicate that a vowel usually silent is to be pronounced, in order to fit the rhythm or meter. Most often, it is applied to a word ending with -ed. For instance, the word looked is usually pronounced /ˈlʊkt/ as a single syllable, with the e silent; when written as lookèd, the e is pronounced: /ˈlʊk.ɨd/ look-ed). It can also be used in this capacity to distinguish certain pairs of identically spelled words like the past tense of learn, learned /ˈlɜrnd/, from the adjective learnèd /ˈlɜrn.ɨd/ (for example, "a very learnèd man").

Solution 2:

This line is from Shakespeare's Sonnet 30. Shakespearean sonnets are 14 lines of iambic pentameter, so each line needs ten syllables. If the -èd was not pronounced separately then there would only be nine in this line, which would break the metre. The grave accent is a help to the reader.

Incidentally, it is not stressed here.

A line earlier in the same sonnet is

And moan the expense of many a vanish'd sight

where the apostrophe is a help to the reader not to pronounce the -ed separately.