In a book about the power of thinking the author writes:

The mind becomes powerful, growing in strength continually, through

meeting a difficulty, in thought; moving forward towards the difficulty, in

thought; and then putting the weight of the mind and will behind it. Then

the "whole man" moves forward, going right through the difficulty to the

other side, victoriously. This generates inward power, that is cumulative,

which, when we come to our difficulty in actual experience, helps us

through it successfully.

What does "behind it" refer to?

And does the "weight of the mind" mean something like the strength of the mind.
Behind it: the author is personifying 'difficulty' - which is approached mentally. Then, the mind and the will get behind it. Think of the difficulty being a rock and the mind and the will teaming up to push it aside, so to speak.

Yes, 'weight' = 'strength' here.

So it means removing the difficulties using the strength of the mind and will?