In Dead By Daylight, when all five players are in the lobby, a 60 or so second countdown begins before the match starts. But it isn't necessary for the whole minute to pass in order for the game to begin. In fact, if all players ready up, then the game will skip the countdown to 5 seconds.

Update: "German" Translation

In Dead By Daylight, vennällfive Pläyers äre in ze Löbby, ä 60 ör sö Secöndcöüntdöwn beginsbeföre ze Mätch stärts. Büt it isnt neceßary för ze whöle Minüte to päss in örder för ze Gäme tö begin. Infäct, if äll Pläyers reädy üp, zen ze Gäme vill skip ze Cöüntdöwn tö 5 Secönds.

When you join a lobby, you'll find that in the bottom right of the screen (below the countdown), there is a button that says "Ready."
Press the Button, and you'll be marked as "Ready."

Venn ü jöin a löbby, yöu find zät in ze böttömright öf ze screen (belöw ze cöüntdöwn), zere is a bütton zat säzs >>Ready.<<

Press büttön.

You'll know you're ready if the button you clicked on becomes an "Unready" Button.
Yöü knöw yöüre ready if ze Büttön u clicked ön becömes an Ünreadybüttön

Here is a semi-informative GIF to help you on your quest to find out how to ready up in Dead By Daylight.

For the people still confused and the person who wanted arrows:

After increasing demand, I have finally made a video guide to help you during these trying times.

Click "Ready."

You should now know how to ready up in Dead By Daylight.