Let's run the smallest useful piece of JavaScript and see exactly what each line does.
<script> opens a block where the browser expects JavaScript instead of HTML. Everything between it and </script> is read as code, not page content.
console.log("Hello from JavaScript!"); sends that text to the developer console a panel most browsers hide by default, opened with F12 or right-click → Inspect. This line changes nothing visible on the page itself.
document.body.innerHTML = "..."; is different: it directly replaces everything inside the page's <body> with new HTML content. This is your first real look at JavaScript changing what a visitor actually sees.
Notice the two lines do two different jobs: one talks to you (the developer, through the console), the other talks to the visitor (by changing the page). Telling these apart is a habit you'll rely on for the rest of this course.