Time on site means how long you spend on a website. It's kind of like how long you spend playing with your toys. If you play with your toys for a long time, it means you spent a lot of time with them. If you visit a website and stay there for a long time, it means you spent a lot of time on that website.
Sometimes people who make websites want to know how long people stay on their website. That's why they track the time on site. They use a special tool to measure how long people are on their website. This helps them to see if people are interested in their website or not. If people are not interested, they might need to make some changes to their website to make it more interesting.
So, time on site is basically how long you stay on a website, and it helps people who make websites to see if their website is interesting or not.