What is impeachment, anyway?
Impeachment does not guarantee removal from office, but it is a serious situation for the president (or other high government official) to find themselves in, and it is the first step toward removal from office.
Technically speaking, impeachment is a formal charge of misconduct levied against a public official by Congress. It is not in any way a verdict or finding of criminality in and of itself. A majority of the House can vote to impeach the president.
How does impeachment work?
In the United States, the House of Representatives has the sole right to initiate impeachment charges against a high official in government.
The process begins with an independent investigation — this could come from the Department of Justice, from Congress itself, or from an appointed special council — which findings for the basis of charges. That evidence is then handed over to the House Judiciary Committee, which reviews evidence and writes up the Articles of Impeachment.
After debate in the House, the legislative body will vote on the Articles of Impeachment. If a simple majority votes in favour of impeachment then the president is considered impeached.