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 the asset management 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 company, assigned to a user ("checked out") on the asset-editing page. You can also add an asset to a particular ticket in the helpdesk (on the ticket page, in the "details" section on the right).