A strangelet is effectivley a lump of matter composed entirley of strange quarks. Quarks (for thoses that dont know) are a group of 6 particles from which all hadrons are composed (ie protons, neutrons measons etc). The six are; up, down, top, bopttom, strange and charm, and there are also 3 colours of each (to allow them to conform to the Pauli exclusion principle), and all the accociated anti particles (ie 36 in all).
The reason why a strangelet would be so dangerous is that it would theoretically absorb all other quarks and convert them into strange quarks too. Since we, and everything else we can see, is composed mainly (my mass) of hadrons, this would be bad.
As far as I know, it is only postulated and has never been confirmed to exist, but i may be wrong.