Could anyone explain to me what cold fusion is please?

Cold fusion is a hypothetical form of nuclear fusion postulated to occur at relatively low temperatures and pressures, as at room temperature and at one atmosphere. We hope to discover how to perform cold fusion for energy uses.

Source:
"Cold fusion." Infoplease.com Dictionary.
© 2002 Family Education Network.

there are two potential ways to get energy from nuclear reactions (fission and fusion).
Fission is the splitting of a large atom into two smaller (but still fairly large) atoms. We've figured how to use this both in bombs and to produce electricity. The problem with this is that the atoms involved are radioactive.
Fusion is the joining of two light atoms into a heavier (but still relatively light). We've figured out how to do this in bombs, but not to produce electricity (it is also how the sun produces its energy). 'Cold' fusion is the type of fusion (with the characteristics MkStfnz notes above) that we would need to perform for controlled generation of power for electricity. The advantage of this is that none of the atoms involved are radioactive. Unfortunately, no one has ever figured out a way to do it (at least not in front of an audience), and many physicists question whether or not it is possible.