A debit spread is when a low premium option is sold and a high premium option is bought. This is the most common type of spread which you need to pay money to put on. Hence a "Debit" to your account. This debit to your account is where the term "Debit Spread" comes from. Anyone with enough money to pay the debit can put on a debit spread. There are no margin requirements and is therefore the most popular kind of spread. Good examples would be a Calendar Put Spread and Options Margin.
You can have debit spreads where the short leg is considered uncovered and hence there *is* a margin requirement.
most don't require the use of margin, but still have do have a margin account to put them on. unless you're only doing cash settled
There is no MARGIN required, but you must pay the debit in cash. That cash reduce margin availability, so you gain nothing.