Most website builders give you one column: the page, top to bottom, and that's it. Cantrip has a second one built in, and I'd guess most people who've built a site here have never touched it. Not because it's hidden, just because nobody tells them what it's for.
It's called a sidebar, and it's a narrow column that runs next to your main content. You fill it with the same building blocks you'd use on any page: a text block, a testimonial, a photo, a small contact form. Stack a few and you've got a widget column, the kind you'd see next to a blog post listing recent posts, or next to a service page with your hours and phone number sitting there while someone reads.
The part worth understanding is where a sidebar actually shows up, because you get real control over that. Build one and set it to appear on every page of your site. Or set it to only show on a specific handful, your blog posts, say, but not your homepage. Or flip that around and show it everywhere except a list you pick. You can also build a sidebar for one page only, and that page-specific version always wins over whatever your site-wide rule says. So your homepage can stay clean with nothing in that second column while every other page carries your standard one.
That's genuinely useful once you see it in practice. A photographer could run a "recent shoots" sidebar across every gallery page but skip it on the contact page, where a busy second column just competes with someone trying to fill out a form. A contractor could keep "call now, we answer" sitting in a sidebar on every service page without rebuilding that block five separate times.
One honest limit: a sidebar attaches to a whole page or your whole site right now, not to one section within a page. If you want it to show up only next to a particular block halfway down a long page, that level of control isn't there yet. It's either "this page has a sidebar" or "it doesn't." Not a dealbreaker for most sites, just something worth knowing before you go looking for the option and can't find it.
Every theme in Cantrip currently supports sidebars, so it's not a feature you'll hit a wall on depending on which one you picked. Look for Sidebars in your website settings, and you'll find the same section-building interface you already know from editing a page. Nothing new to learn, just a second place to put it.