Meetings: Once you click on Meetings, it will open a Calendar view, click on the date on which you want to schedule, then you will see a popup in which you can add the details and can schedule any meetings, see below screenshot.
Opportunities: Opportunities in which you can add your new leads and can manage their stages.
(More will be discussed when we go to CRM module.)
Sales: You can manage, add and send sales quotation.
(More will be discussed in Sales Module.)
Invoiced: You can see and create Invoices
(More will be discussed in Sales Module.)