Atmospheric pressure (also called barometric pressure), is the pressure exerted by the weight of air in the atmosphere upon the surface. Pressure values can vary based on high or low pressure systems as well as varying based on elevation. Pressure measurements are invaluable in weather forecasting to help find surface troughs, pressure systems, and frontal boundaries.
There are three commonly used metrics for pressure: 1) surface pressure; 2) altimeter; and 3) sea level pressure.
Sensors measure surface pressure, which depends strongly on elevation. Historically, mercury barometers were used so the unit of measure was inches or mm of mercury (height of the mercury column varying with the atmospheric pressure). Researchers using the metric system use Pascals or hundred of Pascals (hPa). Additionally, there is a now less used unit: the millibar (mb), which is the same as hPa.
The aviation community uses altimeters, that historically was necessary so that they knew their elevation above the ground. Modern planes have gps now to provide that. It is calculated from the surface pressure, elevation of the sensor, and assumptions about how much pressure decreases with elevation (roughly 1 hPa decrease in pressure every 8 m). Traditionally, altimeter pressure has been reported in inches Hg.
Finally, sea level pressure is the most obtuse of the three metrics as it depends not only on the measured pressure, elevation of the sensor, and assumptions about how pressure decreases with elevation BUT also minor location gravimetric corrections and temperature at two recent times of the day (to minimize small daily heating effects). The formula is really complicated, so we (and most people) compute sea level pressure to be the same as what the altimeter pressure would be at sea level (commonly referred to as Mean Sea Level Pressure, or MSLP).
Now, what’s the point of having all these things? You can’t interpret a surface pressure “map” in terrain, as the elevation differences dominate. That’s what leads to sea level pressure to remove the terrain effects and to be able to compare pressure at different locations. If you look at your own barometer, or just look at one station, then all that matters is how any of the pressure metrics vary in time, the variations will all be the same. Looking at a map of sea level pressures around the country (see image below), helps meteorologists to forecast various weather events.