Answer
Level cover provides the same amount of cover throughout the term of the policy, depending on how it was set up at the start. For example, if you chose £100,000 over 20 years and died in the last year, you'd get the full £100,000.
Decreasing cover reduces over the term of the policy and is usually cheaper than the level cover. Some decreasing cover policies reduce in specific amounts, for example, to match a mortgage or to cover an inheritance tax liability.