Spell Slots

If you're a prepared spellcaster—such as a technomancer (see Starfinder Tech Core)—you must spend time each day preparing spells for that day. At the start of your daily preparations, you select a number of spells of different spell ranks, determined by your character level and class. Your spells remain prepared until you cast them or until you prepare spells again.
Each prepared spell is expended after a single casting, so if you want to cast a particular spell more than once in a day, you need to prepare that spell multiple times. The exception to this rule is for spells with the cantrip trait; once you prepare a cantrip, you can cast it as many times as you want until the next time you prepare spells. See the Cantrips page for more information on cantrips.
You might gain an ability that allows you to swap prepared spells or perform other aspects of preparing spells at different times throughout the day, but only your daily preparation counts for the purpose of effects that last until the next time you prepare spells.

If you're a spontaneous spellcaster—such as a mystic—you choose which spell from your spell repertoire that you're using a spell slot for at the moment you decide to cast it. This provides you with more freedom in your spellcasting, but you have fewer spells in your spell repertoire, as determined by your character level and class. When you make your daily preparations, all your spell slots are refreshed, but you don't get to change the spells in your repertoire.

Both prepared and spontaneous spellcasters can cast a spell at a higher spell rank than that listed for the spell. This is called heightening the spell. A prepared spellcaster can heighten a spell by preparing it in a higher-rank slot than its normal spell rank, while a spontaneous spellcaster can heighten a spell by casting it using a higher-rank spell slot, so long as they know the spell at that rank (see Heightened Spontaneous Spells below). When you heighten your spell, the spell's rank increases to match the higher rank of the spell slot you've prepared it in or used to cast it. This is useful for any spell, because some effects, such as counteracting, depend on the spell's rank.
In addition, many spells have additional specific benefits when they are heightened, such as increased damage. These extra benefits are described at the end of the spell's stat block. Some heightened entries specify one or more ranks at which the spell must be prepared or cast to gain these extra advantages. Each of these heightened entries states specifically which aspects of the spell change at the given rank. Read the heightened entry only for the spell rank you're using or preparing; if its benefits are meant to include any of the effects of a lower-rank heightened entry, those benefits will be included in the entry.
Other heightened entries give a number after a plus sign, indicating that heightening grants extra advantages over multiple ranks. The listed effect applies for every increment of ranks by which the spell is heightened above its lowest spell rank, and the benefit is cumulative. For example, slice reality says “Heightened (+1) The damage increases by 1d8.” Because slice reality deals 7d8 void damage at 6th rank, a 7th-rank slice reality would deal 8d8 void damage, an 8th-rank spell would deal 9d8 void damage, and so on.