What does this mean and how dies it translate into a raw number of cms? 8.9 ± 1.4 cm?

In an algebraic sense, you would have two answers.

8.9cm + 1.4cm = 10.3cm

and

8.9cm - 1.4cm = 7.5cm

so your two answers would be 10.3cm and 7.5cm

In geometry however, this may be a measurement that represents a level or error. you have a range of 8.9cm, "more or less". The "more or less" in this instance would be 1.4cm, that would be your margin of error.