Question 1: I interpret 'enzymes can work in either direction' to mean that enzymes can catalyse the joining of substrates together to synthesise a new molecule, as well as catalyse the breakdown of a molecule (here, that molecule is the substrate) into its component parts. Essentially, enzymes can both catalyse the build-up and breakdown of a substrate.
Question 2: Think of an enzyme as a machine that makes bread, substrate as dough and product as bread. If we have too little dough, the machine will not produce a required quota of bread as quickly as if there was a lot of dough present. Conversely, if there was too much bread formed without someone taking it out of the machine, the machine will become clogged (in the same way enzymes are inhibited by feedback inhibition) and it will stop making bread until it is unclogged.