The Dirty: You likely use your earbuds every day, such as commuting to and from work, plus take them with you when you work out, travel, or run errands. Whether they are in your ear or stuffed in a bag, they can accumulate dirt and grime. Bacteria can be transferred from person to person by using someone else's earbuds.
The Clean: Make a simple solution of dishwasher soap and water and use a cloth to wipe your earbuds. You shouldn't use too much water as that could cause damage. Another option is using anti-bacterial wipes to clean them each week.