A binary operator is an operator that takes exactly two operands such as +, -, * etc. Even when you supply more than two operands, the function operates in pairs.
# With two operands.
# This is an example of an infix function since the function comes between the
# arguments.
1 + 2
#> [1] 3
# But it can be written in prefix form.
`+`(1, 2)
#> [1] 3
# With three operands.
1 + 2 + 3
#> [1] 6
# This doesn't work because `+` only accepts two arguments.
`+`(1, 2, 3)
#> Error in `+`(1, 2, 3): operator needs one or two arguments
# Arguments must be supplied pair-wise.
`+`(`+`(1, 2), 3)
#> [1] 6
Created on 2020-07-19 by the reprex package (v0.3.0)