what are expansion slots
How to find out if your computer has a PCI express 3.0 x16 slot - ITPro
PCIe slots are designed to connect expansion cards, which are circuit boards that add new functionalities or capabilities to a computer.
Allegro USB-C PCIe USB 3.2 Gen 2
The Mac Pro has four PCIe expansion slots (one double-wide 16-lane PCI Express slot is occupied by the NVIDIA GeForce 7300 GT in the default configuration, and three full-length PCI Express expansion slots are open).
Presentation On Motherboard | PPT
What is PCI Express and what does it stand for? What does PCIe mean? What types of PCI Express slots and sizes are there?