How htaccess works
An Apache web server reads its main configuration once at start-up. The .htaccess file is the exception: a small text file you place inside a folder, which Apache reads on every request for anything in that folder or below it. That is what makes it useful on shared hosting, where you have no access to the server’s own configuration but can still change how your site behaves.
What it controls is mostly traffic and access. It can send one address to another, force every request onto HTTPS, settle whether the site answers on the www version or the bare domain, protect a folder with a password, deny access to a file, set cache headers, or point missing pages at your own 404 page. On a WordPress site it also holds the rewrite rules that turn tidy URLs into something the software can answer.
Why htaccess matters
Most of the technical decisions that affect search sit in it. A redirect written here returns a real 301 before any code runs, which is cleaner and faster than a plugin doing the same job later. Choosing one canonical hostname and one protocol, and redirecting everything else to it, prevents the same page existing at several addresses and splitting its signals.
It is also the file that quietly decides whether a migration succeeds. When URLs change, the old ones have to keep working, and the mapping usually lives here. Getting it right is a large part of ordinary technical SEO work.
Where htaccess goes wrong
The first trap is that it may do nothing at all. It is an Apache feature, honoured by LiteSpeed too, but Nginx ignores it completely. Plenty of sites carry a carefully written file that has never been read, and plenty of guides hand out rules without saying which server they assume.
The second is fragility. A single mistyped line takes the whole site down with a server error rather than failing quietly, and because the file is read on every request, a long list of rules adds work to every page view. The third is accumulation: redirects added over years without review turn into chains, where one address points at another which points at a third, and sometimes into loops that make a page unreachable. Editing it through a plugin’s interface makes this worse, because nobody keeps a record of what was added or why.
How to act on it
Keep a copy before you touch it, and make the change when you can test immediately rather than at the end of the day. After every edit, load the home page, a deep page and a deliberately wrong address to confirm you get a page, a page and your 404 respectively.
Write redirects to the final destination rather than to another redirect, keep the WordPress block that the software manages separate from your own rules, and add a short comment above each rule saying what it is for. Review the file whenever you migrate or restructure, and delete rules whose reason has expired. If your host runs Nginx, do the same work in the server configuration instead and ignore any advice that assumes this file exists.