What does "compression of spreads on high-yield debt" mean?

Question

What did Federal Reserve Chair Janet Yellen mean when she said the following?

"We’ve also seen the compression of spreads on high-yield debt, which certainly looks like a reach for yield type of behavior."

Source

Answer 1

Investopedia does a perfectly fine job of explaining it:

The percentage difference in current yields of various classes of high-yield bonds (often junk bonds) compared against investment-grade corporate bonds, Treasury bonds or another benchmark bond measure.

So ... if a lot of people are buying junk bonds (i.e. reach[ing] for yield), those bonds' prices go up (due to supply-and-demand) and their yields go down (by definition), meaning the difference (spread) between investment-grade bond yields (which have presumably not changed) and junk bond yields is reduced (compressed).

Therefore, the compression in the yield spread means that a lot of people are so hungry for high returns that they're ignoring the high risk of junk bonds and buying them anyway.