The simple answer is that a toner enhances or subdues colour in the hair. It is not something that we are putting on to try and fix an error.
When we lighten hair we are removing the hair’s natural black/brown pigments and exposing its natural undertone which is red, orange and yellow. This is only really visible when the hair has been lightened.
We all have these pigments in our hair; the darker the hair, the more red/orange it has and the lighter the hair, the more yellow is has.
When someone wants to be a cool ashy blonde, we have to lighten the hair to a pale yellow first (if we lift past this point the hair will become brittle and most likely break). This warm undertone will always be there.
To get the desired colour, we must use a toner. This is actually just a colour that is designed to deposit the tone you want whether this is ash, gold, copper or even pink!
Achieve the colour you want usually consists of two things. Firstly, we must remove the natural pigment we don’t want and deposit the tone we do want.