TTFB stands for Time To First Byte. Time To First Byte is the measurement of time elapsed upon which the first byte of your web site or application arrives at the requesting client/user. It is a metric used to determine network speed between the client and your web infrastructure. The lower this number is, the better. This number can affect PageRank and other data points used by search engines.

Further Reading:

Wikipedia: https://en.wikipedia.org/wiki/Time_to_first_byte