A hygroscopic solid absorbs moisture from the surrounding air. Lyophilised peptides, with their high surface area and polar groups, often are. Absorbed water adds weight, which skews weighing, and it can speed degradation such as deamidation.
A sealed container taken from cold storage is normally allowed to reach room temperature before it is opened, so that moisture does not condense on the cold solid.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.