You use different tags to format your text as larger text, smaller text, bold text, italic text, etc.
Tags are like commands.
If you want a line of text to be bold, you place a tag at the exact point you want the bold lettering to start and another tag where you want the bold lettering to stop. If you want a word to be italic, you place a beginning italics tag at the start of the word and an end italics tag at the end of the word. Learning HTML is learning the code to perform whatever command you want to do. Notice it is exactly the same as the beginning tag except there is a slash in front of the command. The browser doesn't care.