Lyophilization removes water by sublimation. The solution is frozen, then held under reduced pressure so the ice passes directly from solid to vapour without ever melting. What remains is a dry cake or powder containing everything that was dissolved.
Research peptides are supplied this way because a dry solid is far more stable in storage than the same material in solution, where hydrolysis and aggregation proceed steadily even under refrigeration. The trade-off is that the vial has to be reconstituted before use.
Two practical consequences follow. The mass on the label is the nominal peptide content, not the mass of the cake — counter-ions and residual moisture contribute their own weight, which is what peptide content describes. And the cake itself occupies volume, so adding 2 mL of diluent yields slightly more than 2 mL of solution. For milligram quantities in millilitres the difference is small, but it is systematic rather than random: it always biases the calculated concentration high.