There are two levels of interpretation:
Basically "don't think of reward" means two things in material level -
- Don't lose hope and enthusiasm even if you know that the probability of failure is higher than that of success
- Don't become over confident and hasteful just because you know that the probability of success is higher than that of failure
We should first understand that the rewards themselves are of two types:
- Short term reward - which extends to this birth or at max a couple of births (Preyas). It is temporary, uncertain and have side effects. Basically it is material pleasure and binds us to this world.
- Long term reward - which extends beyond the cycle of births and deaths and which becomes part of our happiness even in Moksha (Shreyas). It is certain, permanent and have no side effects. It is basically progress us in the path of liberation and results in the grace of God.
It is important to note that Arjuna is only interested in Shreyas as seen in shloka 2.8.
यच्छ्रेयः स्यान्निश्चितं ब्रूहि तन्मे
शिष्यस्तेऽहं शाधि मां त्वां प्रपन्नम्॥ २ - ७॥
So "don't think of reward" means "sacrifice the short term reward and aim for the long term reward".
3. Practical and straight forward meaning
We don't have control over the outcome of our work and we are even not authorized to either give it or demand it. Its upto the sovereign God if, when and how we receive the reward. Will we surely get the reward if we put our 100% effort? There is no guarantee. Hence we are bound to get disappointed if are doing work for the passion in reward rather than passion in work (in case the outcome is not in our favour).
So be mentally prepared not to break down after the results are out, whether it is success or failure; and instead enjoy the work while you are on it without putting yourselves into mental pressure!
Sounds like facebook gyan but that is true according to Geeta too!