What exactly is meant by 'maintainable HTML code'?

The word ‘maintainable’ in reference to ‘HTML’ code has shown up several times. I wondered what the phrase ‘maintainable HTML code’ means exactly. Does it mean your HTML code is easy to edit or something along those lines?

it applies to anything program related:

if you have a website the size of codecademy, we are talking many files and 10,000+ lines of code easily. You need to ensure the files are properly structured, the code is readable (indent is part of this), proper design.