For the purposes of business accounting, expenses are deductible against taxable income. Common expenses are rent, salaries, advertising, travel, and so on.
Questions arise because some businesses have trouble distinguishing between expenses and purchase of assets, especially with development expenses. When your business purchases office equipment, if you call that an expense then you can deduct that amount from taxable income, so it reduces taxes.
For more, check out: The 7 Key Metrics Every Business Owner Should Monitor.