`||` is the OR operator. It executes the command on the right only if the command on the left returned an error. See https://unix.stackexchange.com/a/24685/23305.

I think your example isn't correct bash though. The part to the right of `||` is missing the `echo` command:

 > true && echo "worked" || echo "didn't work"
 worked
 > false && echo "worked" || echo "didn't work"
 didn't work
 > false && echo "worked" || "didn't work"
 bash: didn't work: command not found