I'm not a constitutional expert by any stretch. But, let me explain it this way.
In many criminal cases, there is a grand jury that is brought together to decide if the person should be indicted on what they are being accused of. Then, if they decide they should be indicted (Impeached) it then goes to a jury trial where both sides present their cases to the judge and jury to decide guilty or not guilty.
This is very similar. The house, in essence, is the grand jury. The Senate is the open court case to decide guilty or not guilty.