What's a keyword?
Think of a job application system as a giant search engine that will match the words in your application with the terms used in the job description to provide the hiring manager with the best matches. Many systems evaluate the content of an application and give candidates a score to tell hiring managers how much of a match an applicant meets what they are looking for. The problem is that this means small differences in terminology and buzzwords may mean your qualified applications gets overlooked entirely. If the company calls it "digital marketing" and you say "online marketing" you risk not being flagged as a match. So, in order to get a higher match and have a better chance of having your resume seen by a human, you need to be sure that your application uses the correct keywords.
Finding the right keywords
The easiest way to find the right keywords is by reading the job description. But, when you read the job description, you're still thinking like a human and these application management systems are computers. It can be hard to tell which words are the most important and which exact terms need to be included. That's why I recommend a trick: use a word cloud generator. Word cloud generators analyze the words you input and create a visual representation of the most commonly-used words within a document. There are a number of free word cloud tools online, all you have to do is google and then copy and paste the job description into a word cloud generator.
For example, I took this old internship posting off of this site and put it into a word cloud generator. Here is the result:
Now it's really easy to see which words are the most important words in the job description.
Using the right keywords
Once you've made your word cloud, make a list of the most-often used words and go through your resume and cover letter to see where you can use something on your new list of keywords. For example, with the above job description and word cloud you should try to mention Facebook, Twitter, communication, media, Instagram, and events (GRAMMY and recording are most likely part of the name of the company and not the job description since this internship was with The Recording Academy). This will ensure you have as many important words as possible in your application materials and that you're using the same language the application system will be looking for.