Someone walked by, singing a tuneless song.是什么意思？

Someone walked by, singing a tuneless song.

有人走过，唱着一首不成调的歌。

someone

pron. 某人，有人；n. 某个人；

例句
This was clearly a failure by someone who thought he was above failure...
这显然是认为自己根本不会失败的人所遭遇的一次失败。

walked

v. 走，步行，散步( walk的过去式和过去分词 )，（鬼魂）出现，陪伴…走，徒步旅

例句
Her lover walked out on her after she had aborted their child.
她的情人在她打掉胎儿后甩了她。

by

prep. 在…旁边，表示方式，由于，经过；adv. 经过，表示保留或保存时用，短暂拜访；

例句
...expensive make-up that we saw being advertised by a beautiful model...
我们所见的由一位漂亮模特代言的昂贵化妆品

singing

n. 唱歌，鸟鸣，<物，无>振鸣，（机器或系统）技术先进；v. 唱，歌唱(sing的现在分词)，（鸟）鸣，嗡嗡叫，呜呜作响；adj. 唱歌的，音乐般的；

例句
She loved singing as a child and started vocal training at 12.
她从小就喜欢唱歌，12岁开始了声乐训练。

song

n. 歌曲，歌唱（艺术），诗歌，韵文，鸟叫声，鸟语；

例句
The song was banned from the airwaves.
这首歌曲被禁止在电台或电视台播放。