MYSQL: Index keyword in create table and when to use it

What does index keyword mean and what function it serves? I understand that it is meant to speed up querying, but I am not very sure how this can be done.

When how to choose the column to be indexed?

A sample of index keyword usage is shown below in create table query:

CREATE TABLE `blog_comment`
(
    `id` INTEGER  NOT NULL AUTO_INCREMENT,
    `blog_post_id` INTEGER,
    `author` VARCHAR(255),
    `email` VARCHAR(255),
    `body` TEXT,
    `created_at` DATETIME,
    PRIMARY KEY (`id`),
    INDEX `blog_comment_FI_1` (`blog_post_id`),
    CONSTRAINT `blog_comment_FK_1`
        FOREIGN KEY (`blog_post_id`)
        REFERENCES `blog_post` (`id`)
)Type=MyISAM

;