The option pool is shares reserved for future hires. Pool size is negotiated as part of every priced round: investors typically require the pool to be sized so the company won't need to top up before the next round. The standard ask is 10 to 20% of fully diluted shares post-financing, with seed and Series A usually at the higher end.
The pool top-up is added to the pre-money side of the equation, which means the founders dilute themselves to fund the pool, even though the new investors benefit from the future hires the pool will fund. This is the 'option pool shuffle'.