Solving Linear System Using QR Decomposition Help

Question

In my linear algebra class, we were taught about the QR decomposition, where if we let a matrix $A$ be written as $A=QR$, then the linear system $Ax=b$ can be written as:

$QR x = b$.

To solve the linear system, we were first told to:

1. Solve $Qy = b$ for $y$

2. Solve $Rx = y$ for $x$.

Apparently, one can solve for $y$ "very easily" without using Gaussian elimination in $Qy=b$. I am having trouble seeing why this is the case. Could someone explain?

Thanks!