How to interpret aov table for Error:Within from R

#1
I have sleep data where there are 5 groups of people, 4 medications, and 5 different days measured. I ran an aov on in R Studio, but the table came back looking like this:

Error:Group:Meds
Group
Meds
Residuals

Error:Within
Day
Meds: Day
Residuals

What I don't understand is what the Error:Within is reporting. Is it the medication effect changes from day to day OR the day effects change from medication to medication?
 
#3
I have sleep data where there are 5 groups of people, 4 medications, and 5 different days measured. I ran an aov on in R Studio, but the table came back looking like this:

Error:Group:Meds
Group
Meds
Residuals

Error:Within
Day
Meds: Day
Residuals

What I don't understand is what the Error:Within is reporting. Is it the medication effect changes from day to day OR the day effects change from medication to medication?
The AOV reports how much of the total variance is explained by the grouping, and how much variance remains within the groups.

What makes up an appropriate grouping depends on your question.