It is because your website is not responsible. You should use display: flex as much as you can because it moves elements to next line when the websites width gets smaller. As well as using media queries, it basically applies wanted changes when the websites width (viewport) shrinks

video on media queries: https://www.youtube.com/watch?v=2KL-z9A56SQ

video on flexbox (really good) https://www.youtube.com/watch?v=GteJWhCikCk&t=52s