Rei is a bow. And also etiquette and gratitude. When you get in to your dojo, you bow in.
When you get out of the dojo, you bow out.
To bow is to show your respect and appreciation. That is why it is very important to do it right.
In kendo, we have a standing bow and a sitting bow.
In the standing bow, you must be aware of the difference between a bow to your training partner/opponent and a bow to the elders, the higher grade holders and shinzen/shomen (see the video, standing bow).
When you bow, you must appreciate and respect each other. It is a part of kendo.
But without these feelings it means nothing. Understand why we do it and do it properly, so it means a lot.