The main proposal is that there are only relationships (though we can't precisely define what a relationship is).
A main piece of evidence for this is that no-one has ever defined anything.
There are several pages that approach this idea from various angles.
It appears to me that this is one of the larger hurdles to overcome in order to understand what is going on.
Whether a mathematician or not, most people feel they have a pretty clear idea what points, lines, circles, dimensions, trees or humans are. To then be told that none of these words are defined goes against our experience of using these words.
It is pretty tough to take seriously an argument that starts by saying that words have no meaning (while arguing using words).
Words don't have meaning. We perceive meaning in the use of words.
This is a critical distinction. It isn't possible to separate meaning from our perception of meaning. Understanding things (anything) is an inherently subjective experience. If we try to remove ourselves from that experience the whole purpose of understanding evaporates.
The Foundational Crises in mathematics illustrates the problem with trying to give words inherent meaning.