Having 30 amp plug installed at home

My previous home had a 30 amp plug which was very nice, so I'm spending the money to have an electrician I have used on several projects install one. Since my existing panel does not have room, one will be added. He will be pulling a permit.

Can someone tell me in simple layman's terms what voltage/amps are needed on each prong so I can question him on what his plans are? I've read other posts where someone's electrician did the wrong thing with bad results. He has seen the plug, and he's not a novice.

I know virtually nothing about this topic, so I'd appreciate the "For Dummies" version.

Thanks in advance.