SIMM stands for single inline memory module. Before you try to understand what a SIMM is, you should read and understand RAM and memory.
A SIMM is a little flat piece of plastic, usually about as long as your finger, that holds a number of memory chips. This plastic card is actually a small printed circuit board. When you decide it is time to add more RAM (random access memory) to your computer, then you buy SIMMs. Make sure you buy the SIMMs that fit your computer, both physically and with the right amount and size of memory chips. In most computers you can open the box and install SIMMs yourself (first call your power user friend). They just pop into the little slots that are made for them.