by bottleneck we mean weak part that makes otherwise capable PC, have less performance.
each game/program uses cpu/gpu/ram in different proportions so thats why people keep saying gpu heavy or cpu heavy applications.
in weak gpu/strong cpu/a lot of ram combo cpu heavy app will have really bad performance while gpu heavy game might hit 100% of its capabilities.
in general you want cpu > GPU as stutter from CPU being to slow is way worse than constant lower FPS. still when its constantly 10 FPS anything will be unplayable