By clicking the arrow pointing upward in the upper right part of the UI, you open a file browser that allows to select one or more assets on your system. When you click "open", the Pixometry Imagin upload windows opens: 

In the upper part of this window you have a list of selected files. I can select one asset in the list and optionally fill in some metadata already. The fields shown are dependant on the configuration of Pixometry Imagin for your organisation. If you want to apply the text to all assets in the list, use the "Save All" button. Here you can also select a channel, this defines the asset collection into which the assets will be uploaded. It is also possible to add them directly to a project