A check is a term used to describe a crack that runs through a board, usually running lengthwise. It is usually caused when lumber dries out too quickly. Adding a UV protective stain can help protect decking and rails. Checking often affects pressure treated framing materials as they are usually sold very green and are put under a lot of stress as they dry out in the sun. Thicker members like 6x6 posts tend to dry out unevenly and are especially susceptible to extreme checking.
Wood reacts differently in the various climates across the US, and different species and grades also have differing appearances. Straight from the lumberyard, lumber components likely already have some cracking and checking present. Lumber grading procedures evaluate these conditions piece by piece and rate the lumber accordingly. A grade one lumber is of better quality, and thus great structural performance, than a grade two, for example. When it comes to posts, grade two is most often accepted and the market norm.
Exterior environments have fluctuating moisture content in the air and materials. Whether in a dry or wet climate, there will still be annual variation. With the typically large cross-sections of posts, commonly 4x4 or 6x6, the lumber does not dry uniformly. As the wood dries, it shrinks, and the shrinking often causes existing cracks to open or twisting to occur. This is normal and structurally acceptable in most cases.