As elbert has said, GPU's work on the number of frames per second they can output to your monitor. So for example, say you had a top end GPU such as the Nvidia GTX 1080, and a 1080p, 144Hz (refresh rate) monitor with a powerful CPU and lots of RAM and you were playing Battlefield 4....it is more than likely that your GPU could output 144 frames per second to your monitor. This would result in extremely smooth gameplay with no stutter or lag.
If however you had a bottom end GPU (say an Nvidia GT 710) and weak CPU (lets pick on AMD an go with a Phenom II 925) and were trying to play BF4 on the same 144Hz monitor, you would notice stutter, lag and issues with low frame rates as the combination of GPU and CPU cannot render and output frames to match your monitors refresh rate.
Hope this clarifies things.