A token is an element of the JSON object: either a key, a value, an hash-table or an array. As an example the char json[] on the top of this page contains 12 tokens (don't forget to count 1 for the ...
The more tokens you allocate, the more complex the JSON can be, but also the more memory is occupied. Each token takes 8 bytes, so sizeof (JsonParser<32>) is 256 bytes which is quite big in an Arduino ...