Proofs. Axiomatisable structures. Functions (maps). Continuity. Sets.
But that's highly subjective, as any answer on your question has to be.
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I am someone in the world that you don't know of (my mom says I'm not allowed to expose myself) well here is my answer: Archimedes is the Father of Mathematician and he contributed a lot in mathematics. He gave the basic concepts of Calculus. Moreover, he also gave various other mathematics concepts. Most of the people think that Euclid is the father of mathematics but they are wrong because Euclid is the father of geometry.
For most people, they are the starting point of their journey into the world of numbers, and mathematics.
The answers will of course depend entirely on the questions of which none have been given.
Symbols are important because they have meaning to human beings. Letters and words are symbols of various concepts arranged in a way that transmits meanings. Numbers are other symbols representing concept. Musical notes are symbols as are pictures in a gallery.
Marin Mersenne (1588-1648) was a monk in France who studied science, especially mathematics. He made some important discoveries in mathematics and musical theory. Some people suggest that his most important contribution to science and mathematics was his correspondence with other scholars. His work on prime numbers of the form 2n-1 resulted in their being known as Mersenne primes.