The president cannot be indicted?
The issue isn't settled, but a bunch of people in legal professions believe this. The idea is it prevents people bogging down the president by selectively overprosecuting shit for political purposes (Brazil might be an example of this currently), especially since the whole process is so slow and such a drain on time and attention. Plus the Constitution provides for impeachment.
This is partly why some people expect Mueller to release a report if he believes Trump has committed any crimes. Mary Wheeler (whose opinions I steal wholesale on these matters) has been saying that Mueller could just include accusations against Trump in the indictments/pleas of other defendants, which is what just happened with Cohen.