The details.
What can I safely change myself?
The content: text, photos, your prices, opening hours, a new service, a short announcement. On a well-built site these are edits, not surgery.
If changing a sentence means opening code, the handover was done wrong. You should get a way to edit the obvious things, or a clear promise that someone does it for you.
What should I not touch?
The plumbing: domain settings, hosting configuration, forms and where they send, anything involving code or connected services. A small mistake there can take the site, or your email, down without any visible warning.
The test: if a mistake could go unnoticed by you for days, it belongs with whoever maintains the site.
How do updates work if I don't want to touch anything?
You send the change, it gets made. For a small business site that is a short email with the new text or photo, and the update lands without you logging into anything.
Before you sign up with anyone, ask what an update costs and how it gets requested, so the answer is known before you need it.
What about adding whole new pages?
A new page is a build task, not an edit: it has to fit the design, the navigation and the sitemap.
Treat it as a mini-project. When a topic outgrows its section on an existing page, that is the signal it has earned its own.
Will I break the site by editing text?
Not when the site is set up for it. Editing areas sit apart from the structure, so a mistake stays a typo you can fix rather than a broken layout.
If you are nervous, make the first edits with whoever built the site on the phone. One guided round covers the basics.