What does "Q" mean in the re-stock estimate?

The Q means quarter, so each Q is a three month block:

Q1 : January - March

Q2 : April - June

Q3 : July - September

Q4 : October - December

Due to long production times, supply chain issues, and shipping delays this is the easiest way to estimate a product's arrival. We will keep the estimates as up to date as possible as information becomes available.