Most often, when people use the terms spool or lag, they are talking about spool. Spool is, simply, how much exhaust needs to be flowing through the turbine to reach the desired boost level, and is referenced by engine speed. For example, your engine will flow roughly twice as much exhaust at 7000rpm as it does at 3500rpm. A given car may need to be running at 3500rpm before it is flowing enough exhaust to spin the turbine fast enough to pressurize the intake to 12psi.
Many things will affect the rate of spool in a system. The biggest is probably the size of the turbine. More later…
See also: Lag