Often when people analyze a movement pattern they identify weak links as muscle groups or motor patterns that aren't equal to others. When people squat, someone might say 'you're not firing your glutes' or 'bracing through your abdominals' identifying these muscles as causing the movement to be inefficient or lag in progress.
However, weak links can be more broadly classified as being deficiencies in either endurance, co-ordination, strength, speed, power and flexibility. These should all be balanced in an athlete. Often sub-optimal performances, injuries or even poor technique can be put down as deficiencies in one or more of the training systems mentioned above.
Athletes can often be very biased or have fixated opinions on what they need to do to improve - 'that we just need to train heavier' or 'do more glute bridges,' but sometimes this is not always the case. That's why it pays to have a coach to objectively critique our performances and identify our weakest link, and then build programs to work on this weak area so we can produce a stronger performance next time.
I know because often I've been that arrogant lifter who just wants to keep lifting heavy. It doesn't always produce the dividends it seems. An athlete must be open and willing to take on the constructive instruction of coaches - after all they are there in our best interest for us to become better athletes and minimize our weak links.
An athlete may desperately want to work on building strength in the bench press. However if flexibility is limited and they cannot reach maximal hip extension and thoraco-lumbar extension then the athlete must be committed to improving this. They can improve strength till the cows come home, but if they cannot get in a good position to activate the right muscles due to poor flexibility they will not be able to and progress will plateau. This requires discipline from the athletes perspective. Coaches will often periodize training with the focus being on the weak area, with the final training block being a peaking block being specific to the sport.
Funny thing is this is a never ending journey of constantly finding your weakest link and refining it, until you find the next until all areas are somewhat even. This will inadvertently prevent plateaus and facilitate constant progression in sport.
So what's your weakest link?