This is just a warm-up question. Just answer in two-three sentences which clearly explain the reasons behind your purpose of travel, whatever it is as:
“For business purposes – to negotiate a contract.”
“To visit my mother, who lives in the US.”
“For medical treatment.”
I am going to visit my aunt, whom I haven’t seen for a long time. She has been inviting me over for a few years now, and only now I have managed to find time to apply for a visa and make the trip.”,
“The reason I wish to get a US visa is to visit the country and sightsee. I have been saving for a long time for this trip.”,
“I have gained to right to attend a conference/seminar/training.”, etc.
Answer shortly and clearly, since the time you have with the consular officer is short and you do not want to waste it by talking gibberish, and nor does the interviewer.