There are only two things that have been understood much later about zero, that it is a quantity that in many circumstances should be treated in the same way as any other numbers and that it requires a special symbol in order to implement a positional system for writing numbers.
The concept of zero itself had already been known many thousands of years before the invention of the positional system for writing numbers.
All the recorded ancient languages were able to answer to the question "How many sheep do you see there?" not only with words meaning "one", "two", "three", "four", "few", "many" and so on, but also with a word meaning "none". Similarly for the answers to a question like "How much barley do you have?".
Nevertheless, the concept of the quantity "zero" must have been understood later than the concepts of "one", "two", "many" and of negation, because in most languages the words similar in meaning to "none", "nobody", "nothing", "null" are derived from negation words together with words meaning "one" or denoting small or indefinite quantities.
Because the first few numbers, especially 0, 1 and 2 have many distinctive properties in comparison with bigger numbers, not only 0 was initially perceived as being in a class somewhat separate from other numbers, but usually also 1 and 2 and sometimes also 3 or even 4.
In many old languages the grammatical behavior of the first few numbers can be different between themselves and also quite different from that of the bigger numbers, which behave more or less in the same way, consistent with the expectation that the big numbers have been added later to the language and at a time when they were perceived as a uniform category.