For some words which end in -o add -es to form their plurals. For example, the word "hero" becomes "heroes" when plural.
Forming plurals is fairly easy in English. You could think of the basic rule as being to add -s to make a noun plural, being sure to remember that there are also many exceptions. It might, however, be more accurate to say that the basic rule is that, to make a plural in English, we add -s or -es. The only exceptions to this rule would be those words which do not change in their plural forms (i.e. "deer") and a few words from other languages whose plural has not been Anglicised, which means so thoroughly adopted into English that it obeys English rules. For instance, teachers talk about a "curriculum" or many "curricula" because the word still obeys its Latin rules.
So, if we think of plurals in English as being usually formed by the addition of an -s or -es, how do we know which one to use when? There is a surprisingly simple way to tell which words need an -es added. These are words ending in f, ch, sh, x, s, ss and z as well as vowels such as o and y. If you say words such as "churches" or "dishes" aloud, you will see that they need the extra syllable provided by the -es. In the case of the the letter o, you can imagine that -os would not be pronounced the same way as -oes. The additional e keeps the vowel long. This quiz focusses on words ending in -o. So, get ready to add an -es to the plurals!
Here is an English spelling quiz so you can get to grips with the subject. Be warned...not all plurals for words ending in 'o' require an 'e'.