If used on its own, 'ehhh' is a show of confusion or uncertainty. However, it can also be used as part of a sentence, even multiple times, when immitating a person of inferior intelligence, or someone who is slow on the uptake.
"And Jonny was all like 'Ehhh, I is ehhh, wanting to know if you is wanting to buy my car."
Ehhh? I don't get it. (confusded)
Ehhh? He is amazing...(impressed)
Ehhh? I have to do all these thing by myself? (frustrated)
ohhh yeah sounds