Various versions of this quote can be found in many sources. "If you sow kindness" refers to the idea of going out and planting a specific type of behavior or relationship with others. When you "sow kindness" you are considerate and thoughtful, willing to help others when they need, eager to become involved in working to assist others in accomplishing their tasks. Kindness involves taking care of others, even if it imposes some difficulty or hardship on you.
"You will reap trust" describes the way others react to a person who has consistently shown kindness. When one gains a reputation for kindness based on repeated demonstrations of the qualities mentioned above, that person gains the trust of others as being one who will reliably react with kindness in any situation. So, the complete saying is stating that if you are kind in relating to others, you will gain the trust of those you help.
I think what that refers to is, if you offer kindness to others, in return you will be repaid by kindness and eventually you will begin to trust. Sometimes its scary because you don't want to let your guard down because your afraid of getting hurt so your initial reaction is to not trust others. But when you put your fears aside and reach out to others you begin to develop your feeling of trust again. It's not easy to trust but it all begins with a little kindness :)