What is the right spelling?
You might be surprised to know that both spellings are correct. However, they are typically used in different places.
The difference between words such as traveling and travelling (whether or not you use a double letter) is the difference between British English and American English. When you are in the United States, you typically will not use a double letter when you add an ending to a verb. That means that Americans will write traveling.
They will also write traveled or modeled. For other endings, such as changing travel to traveler, it is similar. Americans will not use a double letter.
For almost everyone else, the double letter (ll, for example) is the custom. In the United Kingdom, Canada, Australia, New Zealand, etc., they will typically use the British spelling. So, they will write travelling, modelling, and travelled – each with two l’s.
There is nothing that is changed with regard to the meaning of the words. They have exactly the same meaning. However, be aware that if you use one custom in something you write, you should use that same custom throughout the paper. Do not spell the word traveling in the beginning and then travelling at the end.
In addition, words that already end in a double letter are the same in both versions of English. You should not say spiled, for the past tense of spill, because that is not a word. Around the world, the correct word would be spilled.