Tempo is how fast a game of Netrunner is progressing, and can mean slightly different things depending on context.
Sometimes it is used to describe the overall pace of a game, to describe which side is controlling the speed of the game. A Corp Rush deck would speed up the Tempo of the game, while a Corp Glacier deck would slow the Tempo down.
Other times it is used to describe impact of individual cards on the next turn or two. In this case, a card that provides long-term benefits at a high initial install cost would be said to have a negative attribute of causing 'loss of tempo' when installed (although such cards often dramatically improve the player's Board Position once they have recovered from the install cost).
One of the biggest dangers of a Runner card that causes a loss of tempo to install is sometimes it can open a Scoring Window for the Corp over their next two turns.
Also, Tempo loss is one of the subtle benefits of dealing non-lethal amounts of damage to Runners. Every card lost is also another click lost in needed to draw a new card to replace it. This concept is important to a Corp strategy known as Click Compression.