A memory slot, memory socket, or RAM slot is what allows computer memory (RAM) to be inserted into the computer. Depending on the motherboard, there may be two to four memory slots (sometimes more on high-end motherboards) and are what determine the type of RAM used with the computer. The most common types of RAM are SDRAM and DDR for desktop computers and SODIMM for laptop computers, each having various types and speeds. In the picture below, is an example of what memory slots may look like inside a desktop computer. In this picture, there are three open available slots for three memory sticks.
When buying a new computer or motherboard, pay close attention to the types of RAM the memory slots, so you are familiar with what type of RAM to buy for your computer. It is also important to note how many available memory slots are available in your computer. It is not uncommon for computers to have all memory slots occupied, which means if you wanted to upgrade your computer memory some or all of the memory would need to be removed first.