The tax collectors were asking for payment. Neither Jesus nor his disciple Peter had the cash. Jesus asked Peter, “Of whom do the kings of the earth take custom or tribute? of their own children, or of strangers?” Peter answered, “Of strangers.” Jesus replied, “Then are the children free” (see Matthew 17:24–27).

Some Bible scholars believe Jesus felt that he, and possibly Peter as well, should be exempt from the tax, which was intended for the upkeep of the Temple in Jerusalem. In addition, some historians say that Pontius Pilate, the Roman governor who later ordered Jesus’ execution, was appropriating Temple funds to build an aqueduct to bring water into Jerusalem. Public anger over this use of funds earmarked for something else caused riots. 

To sum up: The tax collectors were asking Jesus for money he did not have to pay a tax he may have felt he didn’t owe that was possibly going to be misappropriated anyway.