Imbibition: The adsorption of liquid, usually water, into the ultramicroscopic spaces or pores found in materials such as cellulose, pectin, and cytoplasmic proteins in seeds.*
When you put dry seeds in water or moist soil, they begin taking in water and swelling up in preparation for germination.
*Allaby, M. (2006). A dictionary of plant sciences (Revised ed.). Oxford: Oxford University Press.