Expenses are costs that team members initially pay out of pocket to be reimbursed by the company. These might be recurring (e.g., internet, co-working space, etc.) or one-time expenses (e.g., travel, office equipment) and, are generally not taxed.
What can and cannot be submitted as an expense, will depend on the employment type (Oyster Direct or Indirect, through an Employer of record) and on the country of employment. This is because the expense reimbursement process may vary by country, due to local laws and regulations.