What is this slot called and what is it for on a Pentium 1 motherboard?
Here I give you the Asus P/I-P55T2P4 rev 2.1 board:
That's CELP! Card edge low profile!
It's a type of slot (or socket) used for expanding the L2 cache on some early–to–mid 1990s computers. The card that would fit inside it was called COASt! Cache on a stick! The stick would be populated by SRAM memory chips.
The standard was originally defined by Motorola to be between 4.33 and 4.36 inches (110 and 111 mm) wide, and between 1.12 and 1.16 inches (28 and 29 mm) high.
The missing puzzle piece...
It's the L2 Upgrade Cache Expension Slot. Check Manual for details