The anon got it wrong. These are ">" or "greater than" symbols, not arrows, although they look like arrowheads. They are used to signal a quote, which makes the text green on 4chan or inside a quote block on other platforms. In 4chan culture, the resulting greentext is not a quote, it has a meaning of its own.
The actual implication arrow, used in mathematical logic for statements like "if A, then B" or "A implies B", is "⇒" or "rightwards double arrow" in Unicode. Using ASCII characters, it can be written as "=>".