- este (m.), esta (f.), they play as adjectives.
- éste (m.), ésta (f.), esto (n.), they play as sustantives.
Examples (well formed):
- —Yo como esta fruta. —¿Cuál fruta? —Ésta.
- —Yo bebo este licor. —¿Cuál licor? —Éste.
- —Me golpearon con una cosa. —¿Con qué? —Con esto.
If you say "Yo tiro esta", the phrase is incomplete, if you say "Yo tiro ésta" you are making reference to some feminine substantive which is implicit.
And you can use the neutral one, when you are talking about something without naming it. If you say "una cosa" (a thing), "algo" (something), etc. or you never name it, you can use it, but you have to give some indications (like pointing the object).
—Hola, ¿qué te pasó? —Me golpeé la cabeza. —¿Con qué? —Con esto.
To understand it better, you could use syntactic analysis:
P: *Predicado* (predicate)
N: *Núcleo* (noun?)
OD: *Objeto directo* (direct object)
MD: *Modificador directo* (direct modifier)
Yo como esta fruta.
N V MD N
The previous phrase has no sens without "fruta", "esta" needs to modify something:
Yo como esta ….
N V MD N
But, you can use ésta:
Yo como ésta.
N V N
This "tilde" (I mean whit this function) is called "tilde diacrítica". As @guifa comment, it is falling into disuse in a few words ("sólo", "éste", "ésta", "aquélla", etc.). The online dictionary of the Real Academia Española still recommends the use of it, but the New Orthography recommends avoid it. Still, I think it is a good way to showing the difference here, and to make clear that it not a typo (you don't forget to write the noun).
Anyway, "yo tiro este" is ok if you follow the new recommendation and you are in a context in which you know if the noun is feminine or masculine. I do not use Duolingo anymore, because they do not take note of their own mistakes.