Marginal tax rate is the rate paid on the next dollar of income earned, the top bracket a company or individual falls into. Effective tax rate is the actual average rate paid across all income, total tax divided by total income, which is almost always lower than the marginal rate.
Tax brackets are progressive: income is taxed in layers, with each layer taxed at its own rate, not the whole amount taxed at the top bracket's rate. A company in the top marginal bracket still pays the lower rates on the earlier layers of its income, which is why its effective rate ends up meaningfully below the marginal one.
Marginal rate is the more useful number for decisions about the next dollar: whether an additional deduction or piece of income is worth pursuing depends on the marginal rate, not the average one. Effective rate is the more useful number for understanding what a company actually paid last year as a share of its income.
Add your revenue and website for a full diligence brief, reviewed by a CPA who ran EY's West Coast R&D Tax Credit practice for 13 years.