You entered an art contest. You submitted an example of your art to the owners of an art-related website. A few week later, you get an email telling you that you didn't win. The email's author wanted to make you feel better about not winning. This is what she wrote.
We received over a hundred submissions.
To "receive" something basically means to get it from someone.
The word "receive" is pretty formal. It's used in official documents or in business communication. In normal conversation, "get" is more common:
We got over a hundred submissions.
"Receive" is often used in a pair with the word "give" - there's a famous saying "It's better to give than to receive."
This means "more than (a number)". It's used in both spoken and written English, but in writing it's probably more common than using "more than".
There's a small difference between saying "a hundred" instead of "one hundred". Although both are supposed to mean 100, using the word "one" sounds more exact. Using "a" sounds more approximate.
Of course, this can also be used for other categories of numbers: "a million", "a dozen", "a tenth", etc.
"Submissions" are the things that people submit, like entries to a contest. The ending "-ion" is often used to change a verb into a noun:
submit -> submission
apply -> application
receive -> reception
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