How to read speaker sensitivity rating?

The opposite, 102 db means a sound pressure level of 102db with 1 watt of input, vs. just 78db with the same input. Not necessarily easier to drive, just plays louder with the same power. Harder to drive usually relates to the ohm rating - 4 ohm is a harder load on an amplifier than 8 ohm is.

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Okay, assuming the nominal impedance for both speakers is 4 ohms. Does that mean the one rated at 78 db will play louder than the one rated at 102 db at the same power?

I've seen a few discussions here about the difficulty of some speakers to be driven and I often see sensitivity numbers thrown around.
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Again, other way around. The units on the ratings are db/watt/m. So the 78 dB speaker produces an spl of 78 dB, measured 1 meter from the speaker, when driven with an input of 1 watt. The other would give an spl of 102 dB with the same one watt of input power, measured at the same 1 m distance.