Lights. Please explain to a newbie

All depends on the type of light you’re using. Seedlings don’t need much at all but later in Veg you’ll need more. Look up PAR or photosynthetically active radiation and PPFD. Photons are almost like a shower head of water hitting the canopy. There’s all types of factors to it. But no other plants aren’t actively stealing the photons. If you have a decent LED light and wanna spend get this app called Photone and it gives you a reading in PPFD from your phones camera. You wanna be around 2-300 umols for seedling 500ish for veg 750-1000 for optimal growth in flower