sequential access in a collection is where each item has to be looked at before checking out the next one. This means that in the worst case scenario a search may require looking at every item in the collection.
Unbalanced Trees Edit
Trees are used to allow a collection of objects to be searched quickly. However most search algorithms work best with balanced trees. The Worst case scenario is that at each node only one side is used. This turns the tree into the equivalent of a linked list and any search on it will essentially be a sequential one.