The eye of the needle wooden door entrance with a smaller doorway inside of a larger door. Jesus used a reference to the eye of the needle in Matthew 19:24 when He said "And again I say unto you, It is easier for a camel to go through the eye of a needle, than for a rich man to enter into the kingdom of God'.
What Jesus was referring to was a small door that was used to enter the city of Jerusalem. This was a small doorway inside a larger gate entrance where a camel could only walk through with great difficulty, squeezed through on its knees and have all of its contents unpacked including the rider and any carry on items. It took great effort and was no doubt a humbling experience which is why Jesus used this example.