Carbon Dioxide (CO2) is a colorless, odorless gas produced by respiration; a gas humans breath out.
The detection of CO2 by your Pelican solution tells your Pelican solution when a room is occupied and how "many people" are in that room.
Higher CO2 = more people in the room. 1000PPM normally indicates that the room is at the occupancy required for the maximum ventilation rate to be provided into the space.
Lower CO2 = less people in the room.
Depending on what Pelican equipment is installed at your site, your Pelican solution can either adjust the ventilation rate (how much outside air is being brought into the space) in relation to the CO2 readings and/or enable the HVAC unit's supply fan to circulate the air and bring in additional ventilation.