Songwriter /'sɒŋˌraɪtə/ (n): A normal person who (1) observes details in the world; (2) hears and connects them in a musical way; and (3) writes them down.
An abandoned object, a dog barking at a bird, a faded book, a new school, a conversation on a train, a misheard lyric, a new take on an old phrase. It depends if you pass them by, or pick them up and remember to take them somewhere.
Anyone can do this. Most people do (1) and even (2) from time to time, but not (3). If we work at all of them, all the time, and go back and squeeze the potential out of - and squeeze our feeling into - what we find, then that's when the magic can happen.