In an absolute, unambiguous sense... wrong question.
Symbols are just symbols. Words don't have an inherent meaning. We perceive meaning in words by virtue of our relationships with those words. Where several people have similar relationships to a word they will perceive similar meaning.
In order to convey a new meaning - or to refine an existing meaning, we communicate the relationships that we think are most relevant.
As such, the best way to understand what I mean by 'relationship' is the same way you learned the meaning of all the words you know - through context.
When you first come across a new word, or a familiar word in an unfamiliar context, you will usually be able to get some sense of the meaning from the surrounding context. The more detailed and varied the context, the better you'll understand the intention (a narrow, repeated context gives little extra information compared with a varied set of contexts).
This is the only way to learn the meaning of a word.
It would be amazing if you didn't already have some sense of what you mean by 'relationship'.
1. Everything in this universe is related to every other thing in this universe (including thought that arise in this universe).
2. Many of these relationships are indirect: A is related to B; B is related to C; A is indirectly related to C.
3. Relationships are also differences. In mathematics there is an 'identity relationship' which is the relationship an object has with itself. This is formalised within The Laws of Thought (identity; excluded middle; non-contradiction). However, the Laws of Thought are inconsistent with a relationship centric view. They cannot both hold.
A relationship between A and B specifies that A and B are distinct.
Hmm... nope - not quite right... the only thing that exists is the relationship. A and B are shadows cast by the relationship(s).
4. 'Everything' then is relationships. That is - relationships relate other relationships. A 'thing' is always a relationship or a group of relationships.
5. This isn't as bizarre as it sounds. The status quo of physics consists of around 20-30 'particles' that make up everything else. These particles (quarks, photons, electrons) are already pretty abstract mathematical ideas. Nobody has ever seen a quark - nor even an electron or photon. What we have perceived is the result of a series of interaction. From these interactions we have worked backwards and made a best guess at what might have lead to our observation.
We already think that a tree is fundamentally composed of quarks (and a couple of other fundamental 'particles') - and those 'particles' are something along the lines of particular harmonics of probability waves.
A 'relationship' is a whole lot more comprehensible than A 'quark' (for everybody - including physicists).
6. The major hurdle is not understanding that there are relationships - but understanding that there can be nothing else.