When current electricity was invented first, it was assumed that the electricity was produced due to the flow of positive charges. This positive charge flows from higher potential to lower potential. So, the direction of conventional current is taken to be from higher potential to lower potential or from positive plate to the negative plate of an electric cell. But we know that actually, electric current is the flow of negative charges or of electrons, so the actual direction of electric current is from lower potential to higher potential.
That is from the negative plate to the positive plate of an electric cell. Therefore, the actual direction of electric current is opposite to that of conventional current. The arrow demonstrated in the diagram is indicating the direction of conventional current.