Say you’re in the kitchen cleaning up and find something that belongs upstairs. You take it toward the stairs, but get distracted by the unfolded laundry you see in the living room as you walk by. You may think, “I forgot to do that,” and jump to folding laundry, forgetting that you were heading upstairs (not to mention cleaning up the kitchen).
People with ADHD get distracted because whatever is in their focus in the moment cuts off other, weaker messages. This can happen in mid-conversation, when a word triggers a thought that leads a person to another subject entirely.