KEY expression in MySQL

[kiranpvsr]

CREATE TABLE message (
id bigint(20) DEFAULT '0' NOT NULL,
inReplyToID bigint(20) DEFAULT '0' NOT NULL,
createMonth int(11) DEFAULT '0' NOT NULL,
createDay int(11) DEFAULT '0' NOT NULL,
createYear int(11) DEFAULT '0' NOT NULL,
boardID bigint(20) DEFAULT '0' NOT NULL,
subject varchar(80) DEFAULT '' NOT NULL,
authorEmail varchar(80) DEFAULT '' NOT NULL,
msgText text DEFAULT '' NOT NULL,
PRIMARY KEY (id),
KEY inReplyToID (inReplyToID),
KEY createMonth (createMonth),
KEY createDay (createDay),
KEY boardID (boardID)
);

In the above code, what does the MySQL code "KEY inReplyToID" mean and what is its equivalent in Postgre sql?
Please help.