The link opens that opportunity's page, where you can read the details before committing to anything.
What happens when you press the button depends on two things: whether you are already approved for the opportunity, and how the organization chose to handle new applicants. Both are covered below.
One of two, and it tells you where you stand:
That is up to the organization, and there are a few possibilities:
If a form opens, the organization wrote the questions, so fill in everything they have asked for. A half-finished application is the most common reason people wait longer than they expected.
It lets you pick shifts as part of your application, so you are not applying and then coming back later to schedule yourself.
Two things to know about it:
It depends whether the organization has enabled group reservations on that shift.
If they have, you can reserve for your group and give the guest details in one go. The steps are in How to make a group reservation as a volunteer.
If they have not, tell them how many people you are bringing. This matters more than it sounds: a normal reservation takes one spot, however many people you turn up with, so unless they know your numbers the shift will look emptier than it really is.
It shows as Pending on your homepage and on your My Opportunities page.
You will be notified when it is approved, by email or by app notification if you have the mobile app. If you have not got the app yet, see How do I download the mobile app?
Your homepage asks What are you looking to do today? and offers three actions:
You can also open the organization's own portal from your homepage, which is where their resources and anything else they have shared with you live.