need a help to understand random forest sentences

I'm a beginner who just started to learn R and never lean any computer language before. I was just following some tutorials on website.
Currently, I'm studying species distribution using random forest, there're some sentences that I couldn't understand.


avi_dat <- read.csv(file = "D:/species distribution tutorial/Data_SwissBreedingBirds.csv",
header = TRUE,
stringsAsFactors = FALSE)

avi_cols <- c('Turdus_torquatus', 'bio_5', 'bio_2', 'bio_14', 'std', 'rad', 'blockCV_tile')
avi_df <- data.frame(avi_dat)[avi_cols]



bio_curr <- getData('worldclim', var='bio', res=0.5, lon=5.5, lat=45.5)[[c(2,5,14)]]
bio_fut <- getData('CMIP5', var='bio', res=0.5, lon=5.5, lat=45.5, rcp=45, model='NO', year=50, download=T)[[c(2,5,14)]]

until here, I understood how to download the data in R studio.

bg <- raster('/vsicurl/')
ch_ext <- c(5, 11, 45, 48)

the problem is from here.
why do I need the vector ch_ext?

bio_curr <- crop(bio_curr, ch_ext)

and how that vector can crop the raster brick? that doesn't exist before?

bio_curr <- projectRaster(bio_curr, bg)
bio_curr <- resample(bio_curr, bg)
bio_curr <- mask(bio_curr, bg)

names(bio_curr) <- c('bio_2', 'bio_5', 'bio_14')
bio_curr[[1]] <- bio_curr[[1]]/10
bio_curr[[2]] <- bio_curr[[2]]/10

those sentences are correct since I run it on R and had the same result that tutorial showed. I just want to know what are those sentences mean or why they are neccessary.

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