It means the slots are physically 16x in size but only run at 8x speeds. 16x ports can run at 16x, 8x or 4x depending on the motherboard and/or what other slots are being used.
If you google motherboard, look at the bottom left, and scan up and across. There’s 2 black slots portruding out of the board, they’re somewhat long and stretch across to about halfway the width of the board. The top one is 16x, and the bottom is 8x.
It means the slots are physically 16x in size but only run at 8x speeds. 16x ports can run at 16x, 8x or 4x depending on the motherboard and/or what other slots are being used.