This is similar to Matthew, chapter 22:17, and Luke, chapter 20:22, but slightly different. These Pharisees and these Herodians tried to trick Jesus. They wanted to know what Jesus thought about the Roman tax law. Mark said that they asked him whether it was lawful to pay the poll tax to Caesar or not (ἔξεστιν δοῦναι κῆνσον Καίσαρι ἢ οὔ)? They wanted to know the practical answer about whether they should pay this tax or not (δῶμεν ἢ μὴ δῶμεν)? Rome had an annual personal census tax of one denarius, worth about $1.50 USA, not that much. However, many of the Roman tax collectors were considered sinners. As the political party of the Romans, the Herodians, and the religious Israelite Pharisees were both there. Thus, his answer might offend someone. In fact, some Jewish zealots refused to pay any civil tax to the emperor.
This is similar to Matthew, chapter 22:16, and Luke, chapter 20:21, almost word for word. Mark said that the Pharisees and the Herodians came and spoke to Jesus (καὶ ἐλθόντες λέγουσιν αὐτῷ). They called Jesus their teacher or rabbi (Διδάσκαλε). They said that they knew that Jesus was sincere or truthful (οἴδαμεν ὅτι ἀληθὴς), because Jesus did not show any deference to anybody (εἶ καὶ οὐ μέλει σοι περὶ οὐδενός). He did not regard people with partiality based on their appearances (οὐ γὰρ βλέπεις εἰς πρόσωπον ἀνθρώπων). Thus, Jesus taught the truthful way of God (ἀλλ’ ἐπ’ ἀληθείας τὴν ὁδὸν τοῦ Θεοῦ διδάσκεις). They were buttering him up with these flattering statements about how he was so sincere and truthful, since he had not shown any deference or partiality to anybody.