Hi Tung,
To tie together the help documentation and your example, \textbf i is a logical vector of which your rings_vector is the same class, and \textbf X is a matrix or data.frame of which planets_df is the same class. It doesn't matter so much what you name your objects but there are limits.
i <- c(TRUE, FALSE, TRUE)
X <- data.frame(a = letters[1:3], b = letters[4:6])
X[i, ] # returns rows 1 and 3 corresponding where i is TRUE
#> a b
#> 1 a d
#> 3 c f
apples <- c(TRUE, FALSE, TRUE)
X[apples, ]
#> a b
#> 1 a d
#> 3 c f
Created on 2020-07-24 by the reprex package (v0.3.0)