What it is
The word brand puts people off, because it sounds like a logo and a colour palette. For a person it means something plainer: reputation, made deliberate. You already have one. Anyone who has worked with you could say in a sentence what you are good at and what you are like to deal with. A personal brand is that sentence, chosen by you, stated where strangers can read it, and confirmed often enough that they believe it.
That gives it three parts. A subject: the one or two things you want to be the person for. A position: what you think about that subject that others in your field would argue with. And a voice: how you sound when you say it, which is the part people remember longest and the part most often faked.
On LinkedIn specifically
LinkedIn splits the job in two. The profile is the static half. Your headline is the text under your name, and LinkedIn's own advice is to rewrite it so it promotes your expertise or shows some personality, which a job title does neither of. The About section, LinkedIn's help page says, is where you express your mission, motivation and skills to people who view your profile, and one or two paragraphs is enough. A photo, your experience and a few recommendations complete it; LinkedIn's own checklist is short and worth following as written.
The posts are the moving half, and the half that actually builds the brand. A profile states a claim once. A post proves it, in public, to people who never visited the profile, and a post a reader shares reaches people who have never heard of you. A LinkedIn post runs to 3,000 characters and can be shown to anyone or to connections only; a personal brand is built on posts set to anyone.
The two halves have to agree. A headline that says you are the pricing person and a feed of posts about hiring builds nothing, because the reader cannot tell what to expect from you next.
For a founder, one honest complication
A founder's personal brand and the company's brand pull at each other. Post only about the company and you are a brand page with a face. Post only about yourself and the company gets nothing from the effort. The founders who do this well decide the ratio on purpose: how much of their name is the company, and how much is the person who happens to run it. Sonar asks this question at onboarding, as a positioning stance, and holds every draft to the answer.
The material for the posts is the other complication. A founder has more of it than anyone, and less time to write it down. Most personal brands on LinkedIn stall for exactly that reason, and the fix is a system for turning the week into posts. More discipline has been tried. How it works is Sonar's version of that system: your week, read once, drafted in a voice measured from your own posts, with private people kept out of every draft.