For those reading:
parse is a function that takes a string and returns the parse tree and -3! (gotta love APL) is execute that takes a tree and turns it back into a string.
The parsed syntax it spits out allows you to compose pieces of queries or modify them (there are easier ways such as queries return tables that can always be in inputs to other queries).
Maybe the author would like to add more context?
Oh no, please don't attack others like this in comments here. Even if you clearly mean well (which I'd say is the case here), it can only cause harm.
geocar's comments are consistently excellent (including for explaining the niceties of kdb and array languages to the community) and having fewer of them would make this place worse. Not everything needs to be spelled out, and besides, sometimes there just isn't time to do so. It can be a lot more work!
I did vote the comment up before responding to him as KDB, but I'm not sure why really.