What is our true nature and how can karma yoga unite us to our true nature?

Karma yoga is action performed by putting in 100% effort without bothering about the fruits of action. When the results of action is not the motive behind our action, it brings a lot of liberation and fulfilment. 

For example: We always think that in order to be successful we need to do these 3-4 activities, but we forget to include all the uncertainties that could occur in our path to success. Let’s take a concrete example of a student preparing for his annual examination. He is studying with the aim to be first in class. He figures out that in order to top the class, he needs to study all these chapters, practice all these problems etc. Once he master these chapters he will be first in class. Now, if we look carefully, he missed so many other parameters to top the class such as he needs to be healthy on the day of exam. If he has diarrhoea on the night before the exam and wakes up feeling very weak on the day of examination, very likely he cannot perform as per his own expectation. Or let's say there were many questions out of syllabus and some other student in the class had knowledge on those while he did not, then most likely the other student might top the class. Another factor could be due to some reason there was a mistake while correcting the examination papers which many times he may not even know and that actually caused him not be the first in class. At such situations, he may just be frustrated and angry at the situation, because he could not meet his expectation. Instead, if he just studied because he loved to study, we would just be happy with the new knowledge he gained.

Let's see another example. We plant a seed expecting it to give lot of mangoes and keep watering regularly. Now, for a seed to grow into a tree, there needs to be good sunlight, appropriate temperature, rain and many other factors ie. for the plant to give mangoes, it is dependent on many factors not just your actions. 

That’s why Gita says, your action alone does not ensure the result. It is dependent on many other factors. When we understand this, we realise that the ways of karma are unfathomable. There are so many factors that give rise to result. Hence, Gita says to perform our action without bothering about the result. This keeps us free from deep impressions in sub conscious and can lead to union with one’s own true nature i.e. happiness and peace and love.