So I was going through some popular codes and came across this - eval(unescape(50%20%......).

Question

What is this? It's use and how can I use it in code?
See this here-This is what I'm talking about
eval(unescape('%66%75%6e%63%74%69%6f%6e%20%73%30%38%33%35%37%64%30%63%31%66%28%73%29%20%7b%0a%09%76%61%72%20%72%20%3d%20%22%22%3b%0a%09%76%61%72%20%74%6d%70%20%3d%20%73%2e%73%70%6c%69%74%28%22%31%35%35%39%30%38%30%30%22%29%3b%0a%09%73%20%3d%20%75%6e%65%73%63%61%70%65%28%74%6d%70%5b%30%5d%29%3b%0a%09%6b%20%3d%20%75%6e%65%73%63%61%70%65%28%74%6d%70%5b%31%5d%20%2b%20%22%38%34%30%36%37%39%22%29%3b%0a%09%66%6f%72%28%20%76%61%72%20%69%20%3d%20%30%3b%20%69%20%3c%20%73%2e%6c%65%6e%67%74%68%3b%20%69%2b%2b%29%20%7b%0a%09%09%72%20%2b%3d%20%53%74%72%69%6e%67%2e%66%72%6f%6d%43%68%61%72%43%6f%64%65%28%28%70%61%72%73%65%49%6e%74%28%6b%2e%63%68%61%72%41%74%28%69%25%6b%2e%6c%65%6e%67%74%68%29%29%5e%73%2e%63%68%61%72%43%6f%64%65%41%74%28%69%29%29%2b%2d%31%29%3b%0a%09%7d%0a%09%72%65%74%75%72%6e%20%72%3b%0a%7d%0a'));

Answer 1

As JME said it's bunch of percent characters encoded and written in the unescape function which again converts them back to text .
Eval function just executes it out .