A designated survivor is typically an asignee from the US President's cabinet who is in charge of replacing the President and spearheading government functions in extreme cases where all executives of the Government (President, Vice President, Cabinet Members, etc.) are unable to carry out their function (ie. through sudden death, injury, etc.).
Recently, the US capitol has been bombed by a suspected terror attack during the President's State of the Union Address. The President and Vice President were killed, while most of the higher government officials were injured, hence unable to carry out their functions.
Jopay, the designated survivor, who happens to be the House Secretary for Housing and Urban Development, immediately was promoted as President following the tragic incident. Not less than a week in office, Jopay has received multiple criticisms from US citizens regarding his decision-making as their next President.