There a number of different opinions on what Lemon is about and this is a combination of several separate contributions:
Lemon has been attributed with two meanings; Art and Bono's Mother/Technology. According to the reports, there is a history behind the latter. Bono once watched some old home movies and saw one of his mother, who has been a recurring theme in many of U2's songs, where she wore her wedding (?) dress and the dress was in a yellow tint; i.e. lemon. The song is dominated by technology and his mother. Lemon is supposed to be about all the material things we have today, the technology, but despite all the technology, we cannot have a dead person back. The second half of the song is about how Bono still longs for her and is still searching for her, and turning his money into light to look for her could refer to watching old movies. Technology stands out so much in the song because it is this technology that allows Bono to see his mother. Remember the video that mimics the first motion picture ever made.
Jonas Steverud (Maintainer of U2MoL) (2nd of April 2006)
Based on contributions from Mike Diack Michael.Diackukos.varian.com (contributed before the 20th of February 1998), Erica Sayuri Watanabe zoostationiname.com (21st of January 1999) and George Walker finster27hotmail.com (12th of April 1999)
Lemon is about the moon.
Firstly, they are both round and yellow. Secondly, U2 members are in the video blinded by a bright light during the chorus (the piano part) and in the second chorus we can also see the moon for a few seconds.
Thirdly, the song's lyrics point to it:
It's see-through in the daylight. It's bright and colours the cold grey night. It has heaven (because it's in the sky) and it holds on so tightly (it always follows the Earth). A man had longed for centuries to get close to it, so he had invented things to get closer to it. He watched it through a telescope (he liked to stare). He melted the sand (with rockets in the desert), so he could see the outside world (space).
The moon is his destination and imagination and he feels he's drifting from the shore, because of it's attracting force, and there's also ebb and flow. The moon is very bright at midnight and that's when the day begins. But these days the man's effort to conquer it has come asunder. It's pointless to live there since there's no water. The man looks for something other (Mars).
Blaz Preglau apis38email.si (20th of October 2007)
"Lemon is about the moon" Le Moon, La Luna?
Bruce bruce.dunnncr.com ( 3rd of November 2009)