At first I didn't know what the three squares, dots or circles meant. They just felt right. After doing research I found out that there was a universal meaning, and even more exciting was that it fit with what I felt but was unable to verbalize. Turns out speaking "symbol" is just speaking another language, although it tends to go through the subconscious.
Like any language, there's different degrees of fluency. I found sometimes I'd see a symbol, not be aware of the meaning but would still feel something. My subconscious understood. With practice, or with help from a dictionary, it's getting easier, even addictive, to decode the symbols.
Here's what the dictionary says about trees: symbolize new hopes, growth, desires, knowledge, and life. It also implies strength, protection and stability. Being drawn to this symbol means you are concentrating on your own self-development and individuation.
The first book I found on the topic was Functional Pottery by Robin Hopper. The author has a great section on geometry and universal symbols. Since then, I've chosen a secular dictionary based on psychology and books on art history. Symbolism appeals less to our physical nature and more to what I'll call our spiritual side so some sources will be more religious, or lean towards the occult and you'll want to be aware of that when choosing your reading. If you're intrigued by symbolism, it makes sense you'll enjoy discovering it much more if the source fits with your wold view.