ID uses two types of styles for formatting text, Paragraph and Character styles.
ALL text carries a paragraph style unless you've done something special to reveal the [No Paragraph Style] attribute, which seems pretty unlikely in your case as a new user. Paragraph styles include basic type specifications -- face, size, weight, color, etc. -- that gets applied to all text in any paragraph to which that style is applied. The default paragraph style is [Basic Paragraph] unless you define and choose something else (which you really should, but that's a different discussion).
Character styles are used to change particular bits of text within a paragraph so they have different or additional formatting, so adding a stroke would be something that could be done very easily using a character style. Character styles can be very specific, including all the font parameters, or very general, like just applying Bold to whatever happens to be specified otherwise. The simpler and more general the character style the more useful it tends to be.
There are several ways that you can apply a character style. Select text and apply the style is the most basic (and you can use find/change to do this), but it's not terribly automatic. Nested styles are a way to apply character styles automatically within a paragraph style using some sort of trigger values in the text -- number of words, characters or spaces; or the location of a particular character -- but they don't always work well. You can't use a nested style to find a whole word, for example, by entering the word. Line styles, also part of the paragraph style, are similar to nested dtyles, but they apply your character style to entire lines of text automatically.
Fortunately, there's yet one more way to apply chararcter styles automatically that CAN identify not only specific words, wherever they appear in the paragraph, but can find any text string that matches a defined pattern, suc as any workd that starts with a capital letter, or any digit, but only if it is preceded by a lower case letter, etc. These are GREP styles, and they work similarly to GREP find/change, but they cannot replace content, only apply character styles. They also can be stacked on top of other character styles.
I know you panicked when I said GREP, but don't. To apply the stroke to the word "apples" in your sample above you would define a character style (named something useful like stroke) that does nothing but apply a .5 pt black stroke, then in your paragraph style definition go to the GREP styles section and add a new GREP style. Delete the sample expression and just type apple, then choose your stroke style, and the word apple will be stroked no matter where it appears in your paragraph.
If you need to limit the stroking to that specific sentence, you can do that with a look-behind, but that's a bit more complex and I don't want to get into it if we don't have to yet.
Feel free to give us more details about the text and we can give better advice about setting up GREP styles, if you need them, or plain nested styles if they can be set up to work.