Hello Luis
By default, a user device which is moving from one BSSID to another with the same SSID, will automatically disassociate with one AP and associate with the other. This is performed by the device itself simply based on the signal strength it detects from each AP. It will use the same credentials (if the network uses a password/authentication) and will connect the user to the network. There is no need for additional configuration of either the user device or the APs, this will take place by default.
In such a case, there will be a small period of time (on the order of several seconds) where there will be no network connectivity, during handoff from one AP to the next. For web browsing or email, that delay is no big deal, and will largely go unnoticed.
Such roaming, however, is unacceptable as far as some real-time services go, such as Voice over IP or videoconferencing. For this purpose, the 802.11r and 802.11k amendments to the Wi-Fi standard were added. Access points that support these amendments are able to seamlessly hand off a user device from one AP to the next without any noticeable disruption of service. In such cases, depending on the vendor, this feature will simply have to be enabled on the APs, and supported by the user device in order for it to function.
I hope this has been helpful!
Laz