"Ich esse Äpfel" How do you know that is I eat apples instead of I eat apple in singular?
Singular: Apfel = apple
Plural: Äpfel = apples
In the singular, the word starts with the letter "A". In the plural, the word starts with the letter "Ä". (And a further clue: in your sentence, there is no article preceding the noun - in the singular, there should be an article since "apple" is a countable noun).
PS: In English, most plural forms follow a fairly predictable and regular pattern (an "-s" is added to the noun). By contrast, German has many different plural forms. For this reason, you should try to memorise both the singular and the plural forms as well as the gender of every new word.
Ich esse einen Apfel. = I'm eating an apple. vs Ich esse Äpfel. = I'm eating apples.
Hint: umlaut (¨) in a noun often indicates that it's the plural form (but this is by no means a rule)
der Apfel - singular, die Äpfel - plural. Ich esse Äpfel. - I eat apples. Ich esse die Äpfel. - I eat the apples. Compared to: Ich esse den Apfel. - I eat the apple. Ich esse einen Apfel. - I eat an apple. When you are listening to the words, Apfel sounds like ahpfel, and Äpfel sounds like ehpfel.
Just learn the plural by heart.There are rules for German plural, but since there are so many possible forms, plus the exceptions to the rules: don't even try the rules.Just learn the plural by heart for every word you find.