_: means that it de-tags a variable. So, if a variable is declared as new Float:fMyFloatVar and you use it as _:fMyFloatVar the it removes the "Float:" tag.
For tries and dynamic arrays I would just look at other plugins that use them. If you don't know what a trie is there is a thread in the tutorial forum that explains it (or you could always use google and the wikipedia article helped me too).
The cellarray.inc is documented fairly well and helps. celltrie.inc is not documented at all but how you use them is very similar to cellarray.