Constructing elements
A package you import will expose the constructor function for an element, used to display a new instance of that component in your document. This function may receive some arguments (fields) to configure the element's appearance, and creates content which you can then place in your document.
For example, a package might expose a container element with a single required field, its body. Required fields are usually specified positionally, without their names. It might also have a few optional fields, such as the box's width, which is auto (to adjust with the body) by default. Such fields are usually specified by their names.
Here's how we'd place a container into the document, in order to display it as it was defined by its package:
// Sample package name (it doesn't actually exist!)
#import "@preview/container-package:0.0.1": container
#container([Hello world!], width: 1cm)
// OR (syntactically equivalent)
#container(width: 1cm)[Hello world!]
You can retrieve the arguments you specified above later with e.fields, obtained by importing elembic (or e for short):
#import "@preview/elembic:1.1.0" as e
#let my-container = container(fill: red)[Body]
#assert(e.fields(my-container.fill) == red)
#assert(e.fields(my-container.body) == [Body])
Arguments not specified above (not fill or body) won't be available (at least, outside of show rules), as those will depend on set rules which are not evaluated immediately (until you place my-container somewhere).
If you're repeating yourself a lot, always creating the same containers or with similar arguments, one strategy to make that easier is with a variable:
#let red-container = container.with(fill: red)
#red-container[This is already red!]
However, a better idea for templates and such is to use set rules to configure default values for each field.