A drop-set is a set in which you do as many reps as possible with a certain weight and then immediately lower the weight to "pump out" more reps. Although some think drop-sets are more effective for muscle growth because you can feel more "burn", research shows drop-sets stimulate muscle growth similarly well as straight-set training [1, 2].
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Now, the main benefits of drop-sets are is that they allow you to get in a lot of volume in a short amount of time. So if you have a few sets to go in your training but are short on time, adding drop-sets can help. You'll be able to finish your workout without leaving your last working sets open.
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I suggest you use drop-sets only at the end of your training session and with isolation exercises (like arm exercises, side raises, calf raises etc.). Doing drop-sets early in the workout tires you out for the rest of your training and having drop-sets on compound lifts like deadlift/squat is quite risky.