 Traffic analysis is an attack to discover communication patterns. It can be applied even if messages are encrypted. Assume Alice sent messages to Bob but an attacker in the middle is able to detect this communication. The attacker is now able to conclude that Bob and Alice know each other. Traffic analysis is thus a form of social engineering. The number of messages, their pattern, precise timing, but also their absence can be important information for attackers. For example, the interval between messages can be used in SSH timing attacks and the absence of messages can be used by burglars to determine that you are on holidays and break into your house.