Hi, I think the key here is to think a bit step-by-step.
What will happen when you enter x<=a? You will get a logical vector of TRUE/FALSE values.
Then you take the mean of it. This gives you a proportion since TRUE is treated as 1 and FALSE is treated as 0.
set.seed(12345)
x <- rnorm(20)
# Let's say we want the probablity x <= a where a=0
x
#> [1] 0.5855288 0.7094660 -0.1093033 -0.4534972 0.6058875 -1.8179560
#> [7] 0.6300986 -0.2761841 -0.2841597 -0.9193220 -0.1162478 1.8173120
#> [13] 0.3706279 0.5202165 -0.7505320 0.8168998 -0.8863575 -0.3315776
#> [19] 1.1207127 0.2987237
x<=0
#> [1] FALSE FALSE TRUE TRUE FALSE TRUE FALSE TRUE TRUE TRUE TRUE FALSE
#> [13] FALSE FALSE TRUE FALSE TRUE TRUE FALSE FALSE
mean(x<=0) # this really gives us a proportion because it is a mean of 0 and 1
#> [1] 0.5
Created on 2021-10-12 by the reprex package (v2.0.1)