Jesus is talking to a group of people about what they as individuals should or should not do. All occurrences of "you" and "your" in verses 17 and 18 are singular, but you may want to translate them as plural to match the plural "you" in verse 16. (See: [[rc://en/ta/man/translate/figs-you]])
The hypocrites would not wash their faces or comb their hair. They did this purposely to draw attention to themselves so that people would see them and give them honor for fasting.
# Truly I say to you
"I tell you the truth." This phrase adds emphasis to what Jesus says next.
# anoint your head
"put oil in your hair" or "groom your hair." To "anoint" the head here is to take normal care of one's hair. It has nothing to do with "Christ" meaning "anointed one." Jesus means that people should look the same whether they are fasting or not.