Dynamic random-access memory
DRAM is made up of millions of very small capacitors. Each of these capacitors has a transistor to let energy in and out. The capacitor slowly loses energy, and if the DRAM is not refreshed, in the end one or more of the capacitors will let so much energy out, that for example a 1 will become a 0, so data will be lost or corrupted. Almost all computers refresh their DRAM many times per second.