Let not your heart be troubled: ye believe in God, believe also in me. In my Father’s house are many mansions: if it were not so, I would have told you. I am going to prepare a place for you. ~ John 14:1-2 

The ESV says rooms rather than mansions. The Greek word is moné (μονή, ῆς, ἡ), and means an abiding, an abode. lodging, dwelling-place, room, abode, mansion. This Greek word is only used twice in the New Testament. It is used here in John 14:2, and later in verse 23, where it is translated abode.

Consider its use in verse 23 in the wider context.

He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him. Judas saith unto him, not Iscariot, Lord, how is it that thou wilt manifest thyself unto us, and not unto the world? Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and WE (the Father and the Son) will come unto him, and make our ABODE with him. He that loveth me not keepeth not my sayings: and the word which ye hear is not mine, but the Father’s which sent me. These things have I spoken unto you, being yet present with you. But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you. ~ John 14:21-26 

In chapters 14, 15, and 16 of St. John, Jesus speaks of the coming of the Holy Spirit. Here the Holy Spirit is mentioned in the context of the Father and Jesus making their abode with his people.

In John 7, while at the feast of tabernacles, Jesus had said the following:

In the last day, that great day of the feast (this is the feast of tabernacles mentioned in verse 2), Jesus stood and cried, saying, If any man thirst, let him come unto me, and drink. He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water. (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given; because that Jesus was not yet glorified.) ~ John 7:37-39

Notice the text above says, the Spirit was not yet given because Jesus was not yet glorified. This is what Jesus is talking about when he says, I go to prepare a place for you, in John 14:2

The place Jesus went to prepare for us is not some elaborate mansion in the sky reserved for when we get to Heaven. It was the abode of the Father and the Son in our hearts by the Holy Spirit.

Jesus’s message to all at the feast of tabernacles was the outpouring of the Holy Spirit. The feast of tabernacles was a celebration of the Jews (one of their annual feasts), in which God’s people set up tents and rejoiced in the Lord their God together. This foreshadowed the many abodes in the house of the Father in which the Father and the Son would have their dwelling by the Holy Spirit.