[求助]这几个表的意思

请大家帮忙看下这几个表是什么意思　
谢谢


CREATE TABLE friend_tbl (
uin1 INT UNSIGNED NOT NULL,
uin2 INT UNSIGNED NOT NULL,
INDEX(uin1), INDEX(uin2),
UNIQUE(uin1, uin2)
);

CREATE TABLE message_tbl (
dst INT UNSIGNED NOT NULL,
src INT UNSIGNED NOT NULL,
type TINYINT UNSIGNED NOT NULL,
time INT UNSIGNED NOT NULL,
msg TINYTEXT NOT NULL,
INDEX(dst)
);

CREATE TABLE broadmsg_tbl (
dst INT UNSIGNED NOT NULL,
id INT UNSIGNED NOT NULL,
INDEX(dst)
);

CREATE TABLE broadmsg_content_tbl (
id INT UNSIGNED AUTO_INCREMENT,
src INT UNSIGNED NOT NULL,
type TINYINT UNSIGNED NOT NULL,
time INT UNSIGNED NOT NULL,
expire INT UNSIGNED NOT NULL,
msg TINYTEXT NOT NULL,
PRIMARY KEY(id)
);