PAR is the range within the light spectrum that plants use for photosynthesis. It stands for Photosynthetically Active Radiation and falls between 400nm-700nm (think photons). PAR is technically measured in micromoles, not watts (I used the term PAR wattage generically). A micromole is equal to 602 quadrillion photons (that would be 602,000,000,000,000,000 photons) per square meter, per second. In other words, PAR is the photosynthetic photon flux density of light, per square meter, per second. This differs from Lumens, which are used to measure bulb brightness and watts, which correspond to the power in an electric circuit. Since plants require different wavelengths, when deciding on LED grow lights, it’s the PAR (not Lumens) you should concern yourself with. This is because PAR is a more accurate measurement of what your plants are actually absorbing. I mention this because most LED manufacturers tout the Lumen output of their LEDs.
Keep in mind though that In addition to PAR, you also want to match the wattage to your space. You want a minimum of 32 watts (about 18 true watts) per square foot during flowering. You can get away with half this during veg. This is why you want a light with veg and bloom modes.
I use veg only mode during veg, full spectrum during flower, and bloom only during flush.