The lawyer asks for more clarification. He knows that he is supposed to love his neighbor but asks who is his neighbor.
When Jesus doesn't answer a question with a question, he often answers with a parable (in effect, a longer form of a question). Here, he responds with what we are accustomed to call the Parable of the Good Samaritan.
You remember it--a traveller is attacked by robbers, left half dead on the road. Two different respectable religious professionals saw him but crossed over to the other side of the road to avoid him. Yet, a foreigner stopped to help him and helped him with great generosity.
Jesus asked the lawyer, "Which of these three is the neighbor?" The lawyer knew the right answer. Jesus told him to "go and do likewise."
We might translate Jesus' instruction to mean that if he (or we) came across someone in great need, he was supposed to offer help. But, read Jesus' question again, "Which of these three is the neighbor?" That is, from whom are we willing to accept help?
Can we get our heads around the idea that a foreigner of what seems to be us dubious religious outlook would be willing to do something that people we have always respected think is too difficult to do?