These activities you can find at the beginning of the “Course content”.
The Announcements is a space where only lecturers and MOOC staff can post messages for subscribers. Subscription to the Announcements board is compulsory for all subscribers, but you can choose not to receive notifications for this or other resources by changing your notifications settings (How can I configure my notifications?).
In the General Forum, on the other hand, you and the other course participants can post questions, discuss issues and resolve any doubts by creating discussions.