Phy restart on radio drivers typically means if the hardware (radio) is able to sync to the preamble of a packet which has a stronger signal strength while it is already in the process of decoding a packet.
This is typically useful in noisy environments to achieve a "capture effect", where the stronger packet is finally decoded at the rx. radio. If the radio does not support PHY restart then typically such a reception would result in a collision on the rx-side at the radio.
