Appositives and explanatory phrases are very similar elements within a sentence structure. Their shared trait is that each is separated from the rest of a sentence by commas. However, an explanatory phrase adds information not necessary to the rest of the statement, while an appositive is a word or phrase used to rename or further describe a noun.
In the sentence "Jim went to town, even though it was raining, to get some groceries," the commas bracket the explanatory phrase. If you remove this statement, the sentence still provide the basic, useful information that Jim went to town to buy groceries. In the sentence "Jill, my sister, is very talented," the phrase "my sister" is an appositive because it describes "Jill," the noun. Again, the statement is sensible without the appositive, but the phrase "my sister" adds an important descriptor.