Delta Time or Delta Timing is a concept used amongst game programmers. A user with a slow internet connection or computer is often prone to experience lag

Lag

Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs...

 or slowness. The purpose of Delta Timing is to eliminate the effects of lag or slowness of computers that try to handle complex graphics or a lot of code, by adding up to the speed of objects so that they will eventually move at the same speed, regardless of lag.

You do it by calling a timer every frame per second that holds the time between now and last call in milliseconds. You thereafter use the resulting number (Delta Time) to calculate how much faster that e.g. a game character has to move to make up for the lag spike caused in the first place.