What does the Special Rule - Last Month Rule mean?

"Under the Last Month Rule, if an individual is eligible on the first day of the last month of the tax year (December 1 for most taxpayers), he or she is considered an eligible individual for the entire year. HSA accountholders may utilize the Last Month Rule to make a full HSA contribution for that year. If contributions were made to an HSA based on being an eligible individual for the entire year under the Last Month Rule, the individual must remain an eligible individual during the testing period. For the Last Month Rule, the testing period begins with the last month of the tax year and ends on the last day of the 12th month following that month (for example, December 1 through December 31 of the following year). If that individual fails to remain an eligible individual during the testing period, other than because of death or becoming disabled, he or she will have to include in income the total contributions made to the HSA that would not have been made except for the Last Month Rule. These contributions must be counted as income in the year in which he or she failed to be an eligible individual. This amount is also subject to a 10 percent additional tax.