A "shirt pocket" refers to a small pocket located on the front of a shirt, typically near the chest area. It is meant for holding small items like pens, notepads, or other small objects that you might want to keep easily accessible.
I didn't have a top shirt pocket, so I used to tuck them inside my bra. (scientificamerican.com)
I'm not sure if you can tell in the picture, but it has one small shirt pocket and a perfectly modest, but still somewhat open, v - neckline. (honeywerehome.com)
Big readable fonts, small enough to fit in a large shirt pocket. (gigaom.com)