About <main> Tag
<main> is one of the elements that identifies different content areas. and it makes your HTML easier to read and help with SEO(Search Engine Optimisation) and accessibility.
it shows the major content in the <body> Tag.
There mustn't be more than one <main> element in a document.
The <main> element must NOT be a descendant of an <article>, <aside>, <footer>, <header>, or <nav> element.
Example :
<body>
<header>Gecko facts</header>
<main>
<p>Geckos are a group of usually small, usually nocturnal lizards. They are found on every continent except Australia.</p>
<p>Many species of gecko have adhesive toe pads which enable them to climb walls and even windows.</p>
</main>
</body>
<main>tag is containing the main contents of the document.
Source from MDN