Income-related expenses (Werbungskosten) are legally defined as "expenses for acquiring, securing and maintaining income" (§ 9 EStG) and these professional expenses can be deducted on your tax return. These can include commuting costs, business trip expenses, training/education, professional attire costs, or occupational insurance.
A lump sum of 1,000 euros is automatically observed by the tax office (Finanzamt) on your tax return for income-related expenses with no proof required. If you'd like to deduct more than 1,000 euros, you must provide proof for the additional expenses.
Some common income-related expenses: