Meta tags are short pieces of hidden text on a web page that do not appear in the page itself, but tell search engines and social networks what the page is about — for example, its title and a one-line description.
Think of the label on a tin of food: you do not eat the label, but it tells you and the shop what is inside, so it can be placed on the right shelf and found by the right people. Meta tags do that for a web page — they summarise it for the systems that sort and show it.
They are an important part of SEO: a clear title and description help a page show up correctly in search results, and they also control the little preview card that appears when the page is shared on social media.