What is an executive order allowed to do? What is it not allowed
to do?

An executive order is a signed, written, and published directive from the President of the United States that manages operations of the federal government. They are numbered consecutively, so executive orders may be referenced by their assigned number, or their topic. Other presidential documents are sometimes similar to executive orders in their format, formality, and issue, but have different purposes. Proclamations, which are also signed and numbered consecutively, communicate information on holidays, commemorations, federal observances, and trade. Administrative orders—e.g. memos, notices, letters, messages—are not numbered, but are still signed, and are used to manage administrative matters of the federal government. All three types of presidential documents—executive orders, proclamations, and certain administrative orders—are published in the Federal Register, the daily journal of the federal government that is published to inform the public about federal regulations and actions. They are also catalogued by the National Archives as official documents produced by the federal government. Both executive orders and proclamations have the force of law, much like regulations issued by federal agencies, so they are codified under Title 3 of the Code of Federal Regulations, which is the formal collection of all of the rules and regulations issued by the executive branch and other federal agencies.
Executive orders are not legislation; they require no approval from Congress, and Congress cannot simply overturn them. Congress may pass legislation that might make it difficult, or even impossible, to carry out the order, such as removing funding. Only a sitting U.S. President may overturn an existing executive order by issuing another executive order to that effect.