Hello,
When they are talking about position they mean that when you are in the early positions(ep) pre- flop(means before the first 3 cards are dealt) such as under the gun( UTG is 1st to act after the Big Blind), UTG1 and UTG2 you should not enter the pot with weak and mediocre hand only premium. such as AA-JJ and AKo-AJs
Then you have the Mid positions such as Lowjack(LJ) and High Jack(HJ) you can enter the pot preflop with a little wider range than the premiums
Then you have the Late positions pre-flop such as Cutoff(CO), Button, Small Blind(SB) and Big Blind(BB). Here you can play a much wider range of hands
As for Post-flop. This simply means AFTER the flop (1st 3 cards) is dealt
I hope this explains a bit
Good luck