From SKYbrary Wiki
The N1 Indicator is a cockpit gauge which presents the rotational speed of the low pressure (low speed) engine spool, a speed that is referred to as N1. The gauge is usually calibrated in percent RPM based on an engine manufacturer defined rotational speed that corresponds to 100%. The indicator can be a "stand alone" analogue or digital gauge or the information can be incorporated into the aircraft Engine Indicating and Crew Alerting System (EICAS) or Electronic Centralized Aircraft Monitor (ECAM) systems if installed.
On many jet engines, N1 is the primary indication of engine thrust and determination of a target N1 is an important facet of the takeoff performance calculations. Engine Pressure Ratio (EPR) is used for the same purpose on other engine types.