In LoRa modulation the spreading factor defines how many chips encode each symbol. Raising it by one roughly halves the data rate and doubles the time on air, but the receiver gains sensitivity, so the signal can be decoded from farther away or through more obstacles. SF7 is the fastest and shortest range; SF12 is the slowest and most robust.
The choice has real consequences for a deployment. A device at SF12 spends many times longer transmitting than at SF7, which drains the battery faster and eats into regional duty cycle limits, while also occupying the gateway channel. LoRaWAN networks use adaptive data rate to push each device to the lowest spreading factor that still works, saving airtime for nodes that need it. Different spreading factors are orthogonal, so gateways can receive several at once.
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