A verb is a doing word or an action word.
A cheetah runs very fast.
We can also say The cheetah is running very fast.
When we add the -ing ending to verbs with a short vowel sound just before the last letter we double the last letter.
If the vowel sound is long then we don't double the last letter before adding -ing.
We don't double the last letter if there are two consonants (letters that are not vowels) at the end of the word even if the vowel sound is short.