In case of Hexagonal unit cell we have two sides equal , and two angles equal to 90 degrees(right angles) but the third angle is of 120 degrees.
a = b ≠ c, and similarly α = β = 90 ̊, ɣ = 120 ̊
But in case of monoclinic unit cell we have unequal sides but the two right angles are present.
a ≠ b ≠ c , α = ɣ = 90 ̊ and ɣ ≠ 90
In case of hexagonal the third angle was fixed to 120 ̊ only but in monoclinic it is not fixed, it can be any but not 90 ̊ .