HTML stands for Hyper Text Markup Language. It is the markup language used to detail the content of a webpage.
One of the most important things when writing html code is to remember to always close every tag you open. Tags are indicated by the "<" and ">" symbols.
Every new HTML document must being with the same sequence. The picture below is an example of that sequence. These specific characters and words are a set of instructions that the computer reads and from these instructions, produces a webpage.
This is an explanation of characters in the picture below, taken from www.w3schools.com, a website dedicated to explaining HTML, CSS and other coding languages.

Example Explained

HTML Head