Imagine that your friend makes excellent lasagna, and you decide you want lasagna for dinner. Somehow, you want to get yourself invited into his kitchen.
You may start by walking up to his front door and ringing the doorbell. The doorbell signals to your friend that you want to come inside, and maybe even sparks a chain reaction that results in your friend inviting you into his kitchen. Then you can reap the benefits of your visit!
This is what many viruses have to do. Viruses can’t make their own lasagna—ok, not actually lasagna, but the machinery they need to replicate—so they need to steal someone else’s. And to get at the goods, they somehow have to signal to cells from the outside to be let in.