Nobody is explaining the answer well to the OPs question.   

A premine, means coins were given out before mining started.  That basically in the genesis block a certain amount of coins were distributed to accounts when the chain started. 

In PoS coins, this is typically a 100% premine as all the coins are created at block 0.  An popular example of this is NXT.

Instamine is a little different.  That is when a PoW coin has basically been set up so that the the dev and friends are the first and main miners of the blockchain and therefore reap a high amount of coins early on.  A popular example of this is Dark/Dash.

A fair distribution is often a point of contention in either of these cases with early birds who got a lot for little saying everything was fair or has been redistributed by now so don't worry. 

PoW people try to cry foul when seeing any Instamine or Premine saying said coins are cheating, but we should all remember that Satoshi has mined over 1 million Bitcoins, but Bitcoiners too will tell you it is fair because nobody knew back then it would be worth something.  But the fact is nobody knows anything is going to be worth something until it finally is. 

Some people by into IPO coins and share a large part of the genesis block coinbase only to see that blockchain crash and burn, then they cry it wasn't fair that they lost their money  But if it is successful then the people that come later cry it is unfair for somebody to have so much. 

I guess overall, there is just a lot of crying by lots of people a lot of the times when the term "instamine" or "premine" are brought up.