Using the signal strength (i.e. RSSI) you can get a very rough estimate on the distance (for instance: close, far, further). You can use the function sd_ble_gap_rssi_get() to get the RSSI.
There are several theoretical formulas (you can find a simple formula here) you can use to calculate the RSSI value into meters, but in practice the RSSI signal is too unreliable for any accurate distance measuring. There will always be too many variables affecting the measured RSSI, like radio interference, signal/multi-path reflections, and loss in the antenna layouts. So if you need accurate distance measuring, RRSI is not suited and you will have to look into other technologies.
