A verb is a doing word or an action word.

for example:

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.

clap    clapping

bet    betting

sit    sitting

shop    shopping

rub    rubbing

If the vowel sound is long then we don't double the last letter before adding -ing.

sleep    sleeping

shoot    shooting

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.

jump    jumping

think    thinking