There is a sentence I don't understand in Alan Parson's lyric of the song "The eagle will rise again".
And the days of his life are but grains of sand
As they fall from your open hand
And vanish among the land.
What does exactly the first sentence mean? Why the use of "but"?
First, I should let you know that these lines that you have posted here are my introduction to the lyrics of Alan Parson.
The line that you have a question about: "And the days of his life are but grains of sand." is a simile, which is a common figure of speech, and often found in poetry.
Similes, as you will find from following the preceding link, compare two things that on the surface appear to be different, but to the poet's eye have something shared. In a simile, the two things compared are usually linked by either the word "like" or "as." In the line you question, however, the linking word is "but," which here means "merely" or "just" or "only."
Thus, not only are "the days of his life" here being compared to "grains of sand," but also, the writer, by using "but" (meaning here merely or just or only) provides further emphasis on the (in)significance of the passing of "the days of his life" (which, like sand, "fall from your open hand ...").
By the way, here I can't help but be reminded of the popular American TV soap opera, The Days of Our Lives, which always began with the voice-over: "Like grains of sand in an hour glass, these are the days of our lives." Could Alan Parson have been a fan of the show???
Yes, it's just an old fashioned poetic way of saying "only".