Meaning
While I'm sure there are some statistics behind this1, its generally accepted that the majority of a programmer's time is taken up reading existing code rather than writing new code.
This should make sense intuitively. If you get hired at Microsoft, chances are you won't be given a blank slate to make a brand new thing. Often you will be thrust into existing codebases. To make a change to an existing codebase, you need to understand the exisiting code.
It gets to the point where you might spend a whole day reading a dozen or so files only to make a 6 line change to one of them.
Software Development is still a relatively new field. Some things we take for granted may end up not being true after all. It doesn't help that its also pretty poorly researched. Keep an eye out for that.