The overblind is the big blind. The big blind is twice the size of the small blind. The use of the term overblind is very marginal. 99.99% of the time, you will hear big blind and not overblind. It is interesting to know that this variation of the term big blind exists. The overblind is a bet imposed on the player who will be in that position. The act of the blinds paying their respective amounts is one of the first things to happen in a poker hand.
The overblind pays without seeing his cards
The reason we call them blinds is that the players who pay them (also known as "posting blinds") do so before they even see their cards. They will be the only players forced to pay a preflop amount. In some games, there will be a third, unofficial blind called a "straddle". The straddle is to the left of the big blind and will have to pay double the preflop big blind for the privilege of calling last.