Mobility vs. Flexibility
There's a difference in flexibility and mobility, if you don't think so, then you should go back and read some more.
If you're doing an exercise movement, lets say shoulder press for an easy example, and you're unable to press all the way up because your shoulder gets "stuck" at a certain point of the range of motion seems limited, then you have a mobility issue through your shoulders. It can be caused by numerous things, but at least we've identified the issue of what's going on.
If you were to do the same movement and feel tightness in your biceps as you press up and making those biceps seem as if they're being stretched as you do the exercise, then you have a flexibility issue. Your biceps could be shorter and less flexible which will limit the ability to straighten your arm.
These are just examples and once you determine if it's a flexibility or mobility issue, you can move on to doing exercises, stretches or mobility work necessary to correct the issue.