The red L symbols on a surface weather map indicates lower pressure, but what causes the pressure to be lower in a particular region?

Pressure is the force of the air pushing down on the earth's surface. The amount of pressure will depend on the density of the air above, the height of the mass of air and the vertical motion of the air.

In a low pressure system the air is rising. When air is rising it does not exert as much force on the earth's surface. Think of standing on a scale. If someone pushes up on you while you stand on the scale your weight will decrease. This is because the person pushing up on you is exerting a force that partially cancels out the force pushing down. When air rises it partially offsets the influence of gravity pushing air toward the surface.

In a region of lower pressure the mass will be less also. There will be more air leaving the lower pressure region that entering it. As long as this is the case the surface pressure will continue to lower. A mechanism that can cause this are strong upper level winds that diverge the air aloft.

When the air rises, whether it is from a local scale lifting mechanism or a broad area of low level convergence, expect the surface pressure to lower. If there is strong divergence aloft (jet streak divergence, positive vorticity advection) then expect the surface pressure to lower also.