Teams owners can invite external Guest Users to a Team. These users will have access to all the public Channels within a Team, as well as any private Channels to which they are granted access (you can ready more about privacy concerns with Guest User access here).
Staff may not want Guest Users to see internal confidential information. If you're ever unsure if Guests can view the content of a Channel in Teams, you can click the "i" information icon at the top right of any Channel to see its members. Guests are clearly labeled with the mention of "Guest" after their name throughout the Teams interface, as highlighted in the example below.
Staff can interact with Guest Users similarly to any other staff user, though they do have some functionality limitations (see details here). Guest Users can start and join meetings within the Channels they have access to, and they can collaborate on files similarly to how Staff would with any other Staff members.
Guests will also be able to create and edit Excel, Word, and other Office 365 app program files, even if they do not have a personal Office 365 license. HOWEVER, if they don't have the actual Office 365 programs installed on their computers, they will only be able to create/edit files from within the web app version of the apps, so some functionality may be limited. Guest Users will still see the option to open a document in the Desktop App, but their computer may not have the app associated with the file, and so will not actually be able to open the document outside of the web app.