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A yeast starter grows the cell count to the pitching target. A litre of 1.037 starter wort on a stir plate supports roughly 14 billion new cells; intermittent shaking supports around 7 billion and a still starter about 4 billion. Starter wort is made at about 100 grams of dried malt extract per litre.
Starter volume
Volume = cells to grow / growth capacity per litre
Starter wort
Dried malt extract approximately 100 g per litre gives 1.037 gravity
Continuous agitation keeps cells suspended and maintains dissolved oxygen and carbon dioxide removal, both of which raise the cell yield per gram of extract substantially.
Decant the spent starter wort if it is more than about 5% of the batch volume, since its oxidised character and different profile can affect the beer. Cold crash first to settle the yeast.