This is one of the most common questions that most people figure they have answered. Though it is not that easy ! Form is essential for not getting injured aswell as allowing the muscle group that you are supposed to work on do the actual work ! However if you never increase the weight and go a little off perfect form you will never be able to progressively overload your body to make it adapt and become bigger and stronger. So what does this all mean ? It means that if you are familiar with how to lift and you know what flawless form is, and have been working out for about 2-3 years you should be able to slap some weight on ! You don't have to have 100% clean form to still have good form. You can sway a little with your back CONTROLLED AND CONSCIOUSLY (easier said than done, this requires experience and knowledge). Or you can bend your knees and use your legs to get the bar up (using a bicep curls example) to gain momentum to get it up for the last 1-3 reps or so.
To summarize if you are looking to grow then both weight and form matters IF you know what you are doing. (Just remember to not slap so heavy weights on that you will get back pain on doing a biceps exercise...)
Look up some how to videos on youtube (they do excist on Scott's youtube page) and learn how to lift propperly with light weights. As you progress you can chose a weight range you are confertable with to progressively increase your experience and knowledge about lifting. As you become more of an experienced lifter, now it is time to challenge yourself and slap some weights on so you can struggle a bit and break some serios sweat !
Hope this answered some questions for you guyz and ladiez, and have a wonderfull day !! (WHAT ? Yes ladiez are supposed to challenge themselves with some heavier weights aswell)