An assembly is a group of members of an organization (e.g. an EDA, a policy caucus, a working group, an advisory council, etc.) who meet periodically to make decisions about a specific area or scope of the organization.
Assemblies—often, but not always, properly constituted— hold meetings, some are private and some are open. If they are open, it is possible to participate in them (e.g. attending if the capacity allows it, adding items to the agenda, or commenting on the proposals and decisions taken by this body).
Assemblies are a core feature of Decidim (this website) so it's something worth taking the time to understand.