Join in with the singing. Don't just listen.

"Along" can mean "together with" or "at the same time as." "I'm going to take a walk to the park, would you like to walk along with me?" "Children, I'm going to read this story aloud while you read along with me, silently, in your books." Also, a "singalong" can mean a performance in which a singer wants an audience to sing along with them, and may do things to help them--such as quickly reading each line just before singing it. A famous TV program of the 1960s was called "Sing Along with Mitch," hosted by Mitch Miller, in which the songs were sung in a very plain, straightforward way and, supposedly, the audience watching at home could sing along.

It means to sing if you know the words. It if you don't just sit back and enjoy. Join in whenever you do know the lyrics. So sing when you know the words and listen when you don't