Objects
The values templates read — page, site, resources, menus, pagers and more — with every field and the Go-template name it replaces.
Templates read values: page and site in every layout, a resource returned by get_asset, a menu entry in a loop over site.menus.main, a pager from paginator(). Their fields use snake case: .Page.RelPermalink of Go templates is page.rel_permalink, .Site.Params.Title is site.params.title.
A field that has no value is none. Printing an undefined value is an error in Tera, so give optional values a fallback: {{ page.params.subtitle or "" }}, or check them first: {% if page.description %}.