The ten digits are the foundation of arithmetic (and maths, science, life).
The links below will provide more information ~
“1”, "one", "unity", some single “thing”, is an easy, obvious concept, well known to readers.
The digit "2", "two", duplicate “things”, is similarly well understood by everyone over three years old.
The "trick" of arriving at “3” is key to every other number up to infinity.
After "3", the only problem is associating names and codes to the remaining values up to “9”.
After "9", "nine", a new type of coding has to be mastered: place value. This site is concerned with digits used singly. “10” uses two digits.
We have not yet mentioned “0”, "zero", "nothing", "empty". It is an oddity that does not fit the above scheme. See zero for more.
We have used a huge variety of vocabulary on this page. Learning the similarities and differences between these forms is part of understanding mathematics. This is a warning footnote. Exact meanings need to be associated with some of these words: they will come, later. Mathematicians demand such accuracy. For now: forgive our laxity. When using "words" and "symbols" with infants it is easy to forget that they may have no idea of what you are talking about. That is the key point of this site. You have to use vocabulary, words, concepts, that are known to your "reader", "listener", "pupil", "protege".....