The president as head of the executive branch is the administrator of the laws. He can set policy on how those laws are administered. He is also commander in chief of the military. The military obey his orders.
There have always been long standing restrictions on what the military may say. I do not think there are any new ones. They may speak with care.
The president can't actually make laws. His job is "execute" the law; that's why he's also called the Chief Executive. He manages the executive branch of our government. Those are the civilian employees and armed forces members who actually do the work needed to carry out laws passed by the Congress.
Laws don't cover every detail of how the government operates from day-to-day. In order to ensure that our laws - including the Constitution - are carried out properly and in a coordinated fashion, the president can issue executive orders that instruct them on matters of policy and procedure.
President Obama isn't doing anything new. U.S. presidents have been issuing such orders since 1789!
Presidential orders may be overturned by the courts, but that's only happened twice in our history.