A davinci strucutre is a single variable with multiple parts. Structures in davinci are used in various ways from organizing ISIS and PDS header information into a single davinci variable to creating your own structure which contains various pieces of information.
thm.rectify uses a structure to retain information for how to reconstitute (unrectify) data. Also load_pds uses a structure to store header information in davinci.
See Structure operations for more information.