In Upsales campaigns are a way to work companies and contacts in a group.
For example say you are planning a seminar and want to be able to plan appointments and other activities with all contacts that will attend the seminar.
You create a campaign for the seminar in which you can add;
Once you have sent out the invitation and people are attending you add the contacts to your campaign. Later on when you want to send out a email or plan activities you can find all the contacts based on that campaign quickly.