Like a "thingamajig" or a "whatchamacallit," a doodad is an item whose name you're not sure of. You might say that your sister collects doodads she finds on the beach and keeps them in glass jars.
It's handy to have an all-purpose term for those things you can't quite name, either because you've forgotten or just don't know what to call them. Ask your aunt what she sells in her little shop, and she might say, "Oh candles and soap and various doodads." In Britain, the more common word is doodah, and both words emerged in the early 20th century.