When responding to an Opportunity, it is important to view the details of the opportunity along with any documents uploaded by the Project Owner.
In the Opportunity details screen, you will find a box named Respond to Opportunity.
From the dropdown, you will be able to select your response to the Opportunity. The options you have are Accept & Decline.
When Accepting an Opportunity, you confirm you are capable of satisfying the requirements and have all of the necessary staff, equipment, licenses, etc. in place.
When Declining an Opportunity, you decide to not participate or be part of it.
When making your selection and pressing submit, you will not be able to undo your response. The Project Owner will only be informed if you Accept an Opportunity, and not if you decline it.