Impeachment is a tool the US Constitution provides Congress to punish serious misconduct from the President.(e.g treason or bribery)
For impeachment to take place, the House of Representatives need a simple majority to vote for impeachment. Thereafter, the Senate must hold a trial in which it takes a supermajority (two-thirds) of the Senate to convict the president.
If convicted, the president is removed from office and the vice president would take power. Apart from removal from office, the Constitution allows for one other punishment for a convicted president - disqualification from running in any political office under the US in the future.
Trump is currently facing impeachment charges for the incitement of insurrection that happened at the Capitol.