Translation:You are thinking
Yeah, it can also be "you think". Swahili has a couple of different present tenses, one with -na- and another one with -a- but the latter is rarely used in modern Swahili outside of formal things such as newspaper headlines or things describing events as a simple fact without time (eg. describing what happens in a book, since the plot line of a book or a film is an unchanging thing).
Mnafikiri = You think / You are thinking
Mwafikiri = You think [formal, headliney]