On the desktop version of RotaCloud, at the top of the rota, you will see a row titled Open Shifts (provided your account manager has enabled it). These are shifts that haven't been assigned to anyone.
You may receive notifications when Open Shifts have been added to the rota and are available to claim.
You can claim these for yourself on a first come, first served basis by clicking the shift and hitting Claim Shift.
Once claimed, the shift will be assigned to you on the rota automatically - so make sure you can definitely work it before claiming.
Your employer or manager may also be notified of your claimed shift.
Please note - if clicking an open shift doesn't work your manager may have disabled this feature, or you may not be able to work the role.