Medkits are your EMT bag. Rating 1-3 are the large pocket size, and 4-6 are the small duffle/gym bag size. If you've seen paramedics in real life get out their gear bags on a call, then you've seen a rating 6 medkit sized bag.
Autodocs are basically medical droids from Star Wars, just not as fancy. They are (usually) stationary things, so you bring the person needing treatment to them, unlike medkits that you bring to the person needing treatment.For autodocs, the rating indicates how complex a treatment they can do.
With both, higher ratings help more, so get the highest rating you can.