In accounting, debit refers to a bookkeeping entry that represents an increase in assets, or an expense transaction. To make the concept of debit easier to understand, let us apply it to the use of a debit card.
When you use a debit card, you spend your own money to purchase something. You incurred an expense, so the purchase is reflected as debit. Because you also bought something of value, your assets have increased. On the other hand, using a credit card will result in a liability: After the purchase you will owe the credit card company the amount of the product/service you purchased.
The funny thing about debit, though, is that if cash comes in, it qualifies as debit. At the same time, if cash goes out but it results in a decrease in liability, such as when paying of bills, it is still considered to be debit. Thus, to make things simpler, it will be easier to just define debit as the left column found in double entry bookkeeping. This column corresponds to assets and anything that increases it is reflected as debit.