Okay kiddo, when people say "I knit London" they are usually talking about a technique of knitting where you make a picture or design using different colors of yarn. The picture often looks like a map or landmark in a city, like the Big Ben clock tower in London.
To do this, you start by choosing a pattern or design you want to make. Then, you use a piece of paper with little squares on it, called a chart, to plan out which colored yarns you need to use for each part of the picture.
You start knitting as usual with one color of yarn, then when you get to a spot where you need to use a different color, you switch to that yarn. You keep doing this as you follow the chart until the picture is complete.
This technique is called stranded knitting because you carry the unused yarn behind the working yarn as you knit. It can be a fun and challenging way to knit something special, like a gift for a friend or a decorative piece for your room.
So, that's basically what it means when someone says "I knit London" - they made a knitted picture of a London landmark!