The B-Pillar is one of up to four different types of structural supports that make up a vehicle. Every type has an A-, B-, and C-Pillar. Longer models such as full-size SUVs also have a D-Pillar.
The B-Pillar is a vertical roof support piece that is positioned between the front and rear doors. You can see this one from the outside, but it is covered by a trim. Some models can get the stainless steel version (pictured), which is resistant to UV damage and corrosion. Please keep in mind that this piece is eligible for exterior keypads if you want to add that.
The A-Pillar is located on both sides of the front windshield at the face of the vehicle. Meanwhile, C-Pillar is situated on either side of the rear window for support. All of these units hold the top of the car in place. Essentially, they connect the roof to the body.
In larger vehicles, C-Pillars are behind the rear doors, and D-Pillars support the back window. This is how manufacturers build wagons, crossovers, SUVs, and minivans.