A computer's memory occasionally suffers a bit flip, where a stored value changes by itself, from stray radiation or electrical noise. On an ordinary machine this is rare enough to ignore, but across thousands of servers running constantly it happens often enough to matter.
Error-correcting memory stores a little extra information alongside each value, enough to detect and usually repair such a flip on the fly. It costs slightly more and runs marginally slower, which is why it is standard in servers that must stay correct and absent from most consumer machines.