An entry is the set of answers you provide while completing a mission. A lot of the time, you'll submit multiple entries while on a single mission. For example, imagine getting into a mission all about how you play with your dog! You might be asked to take a 30 second video of yourself & your dog and answer a few questions every time you play together over a three day period. Each time you take a video and answer a series of questions about what you're doing in the video, you're "creating an entry."
If you're participating in a mission that asks you to complete five entries, that means you'll complete the same series of questions five times in different circumstances. An entry can be as short as a single question, and is rarely longer than ten questions.