To answer your question sincerely, Tailwind is a series of utility classes that make it easy to author CSS within HTML (pages or components). Back in the day we had many libraries to do this, the most popular was called Bootstrap. It looks like this:
https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/css/bootst...You'll notice that Bootstrap CSS includes every single utility class within Bootstrap. This is fine, but it could be a lot smaller - reducing page load times for clients and data transfer for servers - if the CSS included by a library was solely the CSS used by one's site, and no more. This is why we web developers use an npm/vite build step generally, even if our out is very plain static HTML/CSS.
"Well why can't I just use Tailwind like I do with bootstrap? I just want to play around with it and not have to learn all that build stuff."
You can: https://tailwindcss.com/docs/installation/play-cdn
You'll also notice, if you search for Bootstrap today, that it expects one is using npm and webpack, parcel, or vite.