The five Metadata Dictionary sections (Biomaterial, Process, Protocol, Project, File) list the current set of metadata fields used to describe datasets in the Human Cell Atlas.
The metadata fields in each section represent different parts of an experiment: A biomaterial (e.g. a tissue sample) can undergo a process (e.g. dissociation) to produce another biomaterial (e.g. a sample of dissociated cells) or a set of data files (e.g. 10X fastq files). The process that was actually executed follows a specific protocol (e.g. a 10X protocol). All of these parts make up the overall project (e.g. Understanding cell types in the human heart).
Click on any of the Metadata Dictionary sections, listed in the menu on the left, to start exploring the Human Cell Atlas metadata fields.