I read over and over about docker, did some tutorials. But when my boss asked me: should we use it? And can you please explain one more time what is it anyway? I simply say, well, it IS like a VM but without the VM. No other explanation stuck for me, and I'm sure it's a wrong description of what is docket, but the messaging so far got at least this part for me as the quintessential "what's that docker thing again?"

It's a VM that doesn't need a full guest OS. It's a sandbox. I'm sure I'm wrong but I can't think of another ELI5 for this.