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Why do Japanese people write wwwwwwwwwwww all the time and what does it mean?

Because, many years ago, the US government set out on a wide-spread anti-Japanese social media movement to prevent American youth from further associating with encroaching Japanese culture through the internet. Any script which contains the predetermined cultural cues is automatically scripted as a single "w" or series of "w"'s.

"w" is short for 「わらう」, (which in rōmaji is "warau") which means to laugh.  It's basically the equivalent of our "lol".  You'll occasionally see it in kanji too as 「笑」 instead of "w" since that's the kanji for the word.