In the Middle
In the middle has two meanings. It can mean a player who acts after some players but before others, or it can mean the pot, which is the money in the middle of the table. It can also mean posting the blind out of turn.
The simple explanation
A player in the middle position is not first and not last. The pot is also called the middle because the chips are placed in the middle. When a new player joins, he may be allowed to post a blind in the middle instead of waiting.
A closer look
In a game with several blinds, an incoming player may be allowed to post the blinds in the middle, out of their normal order, instead of waiting for them to pass. Ask the dealer about the local rule when you join a game. Do not confuse the middle with the middle position, which is a different term.
A practical example
A new player sits down and posts the big blind in the middle to be dealt in at once.
Full explanation
In the middle has two meanings. It can mean a player who acts after some players but before others, or it can mean the pot, which is the money in the middle of the table. It can also mean posting the blind out of turn.
In the middle can mean the position, the pot or a blind.
A new player may be allowed to post in the middle.
The rules vary by room.
Look at the context.