Y class refers to the fare class. Some airlines are different, but here is how CO breaks down the Fare Classes:

A: Paid First Class
D: Paid BusinessFirst
F: Upgrade/Reward First Class
Y: Unrestricted Coach
H: Coach (Usually unrestricted)
K, B V Q & T are all restricted discount coach , in order of cost (K Highest, T Lowest).

Y is sometimes a generic shorthand term for coach.

I hope this answers one of your questions. The BEST way to learn, is to continue to read FT. You will find that the community here is very friendly and helpful.

Happy Flying!