I see in the data sheet of some 8-bit MCUs (e.g. EFM8LB1) that some pins are 5V tolerant under bias. What does this mean?
In this context, "under bias" refers to the voltage tolerance of the "5V tolerant" device pins in relation to the voltage of VIO (the "bias" in this case). Thus, as specified in Table 4.19. Absolute Maximum Ratings of the EFM8LB1 datasheet on page 30, the 5 V tolerant pins are tolerant to a voltage of 5.8 V or VIO+2.5 V, whichever is lower.
Note: VDD must also be powered in this case, since Table 4.19 in the datasheet lists that the VIO voltage should not exceed VDD+0.3 V.