Here is the quote: "Because BH4 is not present in saturating concentrations under basal conditions, it is crucial in regulating TH activity." This is from Fundamental Neuroscience, page 121.
I will rewrite the sentence another way that preserves what a believe is the intended meaning:
There is little enough BH4 under normal conditions such that levels of BH4 are important for TH activity.
BH4 is an enzyme cofactor; in this context they're talking about it used with tyrosine hydroxylase (TH) to make a precursor (L-DOPA) to dopamine.
If there was so much BH4 available at basal (normal) conditions that adding more doesn't speed up TH activity, we can say it's saturated. In that context, BH4 changes don't really regulate TH activity (unless they are huge changes that bring it well below the 'saturated' level).
The sentence you quoted is saying this isn't the case. BH4 isn't present at saturated levels, so you can influence TH activity (and therefore synthesis of L-DOPA) by influencing BH4 concentration.