So, here's a quick explanation, not going into too much detail, but just an overview:
A blog tour for a book is something an author does during the release month of their newest book. Bloggers volunteer to host the author at their blogs on scheduled dates. In fact, the blogger has very little to do other than prepare the post in most cases. Generally, an author prepares what they are going to say and sends it to the blogger in advance of the scheduled date. A blog tour post can be a giveaway; a guest post, in which the author writes a short piece of their opinion on a given topic, sometimes given by the blogger and sometimes created by the author; an interview, in which the blogger might give the author a set of questions to answer; a character interview, in which the blogger might choose a character from the author's book and give the author those questions directed at that character; or a tens list, in which the author chooses their top ten on a provided topic.
Each day of that month, the author will stop at the blog scheduled for said date. They will provide one of the above-mentioned items for the blogger before the date, and the blogger will post it on that day. This provides a variety of publicity for the author as many of the blogs have many followers who will then be able to find out more information about the book.
It's as simple as that. Blog tours seem a little scary at first--even participating as a blogger might seem scary at first. But not only does it bring more publicity to the author, it can also bring more followers to your blog. If you get into blogging and participating in blog tours, it'll only be a matter of time before you'll start participating without even thinking about it.