Development is usually split to frontend and backend parts.
Frontend development stands for user interaction (UI) - it is about checkboxes, preferences, visual design, logics of use and other stuff that forms user experience, since is visible to a user.
Backend development stands for Application programming interfaces (API) - technical capabilities of a program, its building blocks under the hood that are indifferent to shaping the user experience since is invisible to a user.
Fullstack development is a development of a feature that include both UI and API - from technical capabilities to a user interface that interconnects these technical capabilities with some kind of logic.
To avoid the necessity in custom builds sometimes is enough to patch backend API parts, providing necessary capabilities under the hood, and solve UI part with custom addons that utilize provided capabilities.