Dry sherry, a wine fortified with brandy, typically shows up in recipes in small amounts. Fortification gives it a little more complexity—and higher alcohol content—than most table wines, so a little goes a long way in recipes.
If you don’t have any on hand, there are plenty of viable subs. The most similar will be other fortified wines like dry vermouth (not sweet), or madeira—you can use equal amounts of these in place of dry sherry. You can also use hard apple cider or dry white wine, also in equal amounts, and while they won’t bring quite the same depth of flavor, they’ll add sufficient acid and fruity enhancing notes to your dish. A good non-alcoholic sub would be unfiltered apple cider. Remember, too: Because we’re usually talking just a tablespoon or two of the stuff, the substitute won’t make or break the recipe.