Splat, ?:, and Array#* in one line

TL;DR

Three idioms stack up in this line. The splat *["b", "c"] inlines the sub-array into the surrounding literal, giving ["a", "b", "c", "d"]. ?: is a character literal for the one-character string ":". And Array#* with a String argument behaves like Array#join. Result: "a:b:c:d".

The splat operator

["a", *["b", "c"], "d"] # => ["a", "b", "c", "d"]

* expands the sub-array's elements into the surrounding array literal. It is handy when part of a row is only known at runtime, CSV generation for example:

title = "Hello"
authors = ["Author 1", "Author 2"]

[title, *authors] # => ["Hello", "Author 1", "Author 2"]

Character literals

?: is shorthand for a single-character string:

?: # => ":"
?a # => "a"

Array#*

Array#* dispatches on its argument type: an Integer repeats the array, a String joins it:

[1, 2] * 3    # => [1, 2, 1, 2, 1, 2]
[1, 2] * ", " # => "1, 2"

So the full expression boils down to a join:

["a", "b", "c", "d"] * ":"     # => "a:b:c:d"
["a", "b", "c", "d"].join(":") # => "a:b:c:d"
["a", *["b", "c"], "d"] * ?:   # => "a:b:c:d"