乐天 comes from the idiom 乐天知命 ('to be content with one's lot and accept fate'), and describes a person who is naturally optimistic, carefree, and takes things as they come. It can be used as an adjective (乐天的性格) or as a noun referring to such a person (他是个乐天). Unlike the English 'happy-go-lucky', 乐天 carries a slightly more philosophical, almost Taoist connotation of accepting life's ups and downs.