Hello Raka and Jeff,
The simple way of understanding both is that they are used for client roaming purpose.
Smart-roam : It is a set defined signal threshold for client on AP below which AP would kick the client to search for another AP with better signal.
BSS-minrate : This is to set the management frame rate of AP, increasing BSS-minrate means shrinking the coverage area of the AP. By increasing the default bss-minrate, clients must be closer to an AP in order to use the higher data rates, and thus will be more likely to roam when an AP with better signal strength is seen.