What Does It Really Mean To “Like” Something On Facebook?

I was reading a post by a Facebook friend of mine.   He said his dog was very ill and requires surgery.   There were several comments below the post and the photo of the dog, but it also listed a couple of names and said "and 8 others like this".    

I recently read another post from someone who lost a close friend and there will also "likes" for that as well.   It seems very weird to me.  Maybe I don't understand the reason for the "like" on FB.   I can understand using it if something good happens, or if a friend posts a nice picture or says something humorous.

I would use the "unlike", but you can't do that until after you "like" something.   Maybe they can change the "like" to icons of a smiley face, or a sad face.   Seems "like" it would make more sense.