As a wizard you have access to any spell in your spell book. But you can only memorize so many spells per day. So when your wizard wakes up in the morning, (s)he opens their spell book and starts reading, choosing which spells to prepare for the day's activities.
For instance, if your wizard's spell book has the following spells:
Mage Armor
Shield
Magic Missile
Color Spray
but they can only cast two spells, then they need to choose which two spells they will have ready for the day. Do they take Mage Armor and one Magic Missile? Or Two Magic Missiles? Or a Shield and Color Spray? Et cetera et cetera.
Preparing spells is one part chess, one part roulette and one part luck. You need to pick what your arsenal is going to be ahead of time and hope you can apply it to the problems at hand.
In contrast a sorcerer(ess) only knows a handful of spells but can cast them several times throughout the day without having to worrying about any preparation ahead of time.
So if we use the above example, a sorcerer(ess) could cast any of those 4 spells until (s)he reached their limit for the day.
Does that help?