R2D2 is the name of a sneaky computer program that quietly sneaks onto your computer and causes trouble. It's kind of like when your sibling sneaks into your room when you're not looking and makes a mess without you knowing.
Like R2D2 from Star Wars, this program is very good at hiding from you and it's hard to detect that it's even there. It's called a Trojan because it uses tricks to convince you to let it into your computer, kind of like how a wooden horse snuck soldiers into a city in ancient Greece. Once it's in, it can do all sorts of bad things like steal your personal information or spy on what you do on your computer.
So, just like you wouldn't want your annoying older sibling sneaking into your room and making a mess without you knowing, you definitely don't want R2D2 sneaking into your computer and causing trouble! That's why it's important to be careful about what you download and to have good anti-virus software to protect your computer.