"montage is an idea that arises from the collision of independent thoughts, wherein each sequential element is perceived not next to the other, but on top of the other" – Sergio Eisenstein
What I think the first part of this means is that the motion of different shots being put together though not necessarily directly related to one another is the vast amount of ideas of multiple people creating in effect something that conveys a message to the audience, for example a use of montage that is common is for example in the film Rocky where we see Rocky (Sylvester Stallone) training, the beginning of the training is hard while towards the end of the montage he becomes the best, while also taking out a large amount of time it would have taken to show that if not for montage, another would be where two characters for example who are in love having the time of their life and are happy, sometimes this is followed directly by a tragedy or something that halts the happiness they have.
The second part of this quote I think means that within the montage each sequence the one which comes after the other is meant to mean that each shot after each other elevates the meaning of the entire montage, for example the beginning shot is to state what is going to happen for example a boy not being able to ride a bike, while the next him attempting to. This can be used in something called intellectual montage, where too unrelated shots can be symbolic to each other.
Other types of montage are metric (the more traditionally used version), rhythmic, tonal, overtonal and intellectual. In the video it shows and example of montage and adds music to it to fit in with the story.