The "Assets" page keeps track of your company's PCs, laptops, printers, software etc. Assets move locations, get assigned to users, get replaced, etc. You can disable this feature in the admin panel if you don't need it.
Just like helpdesk tickets, assets can have "custom fields" assigned to them. To edit the custom fields for your assets simply click the "custom fields" link at the top.
An asset can be assigned to a user (on the asset-editing page). Or even to a particular ticket in the helpdesk (on the ticket page, in the "details" on the right).