open circuit means the circuit is broken.
like if you connected your dmm probes to a piece of wire then cut the wire - the circuit is 'open' and
the electricity can't flow.
Opposite of open circuit is short circuit
- if the component you are testing has its terminals connected with a wire, the electricity flows down the short path i.e. path of least resistance. the energy would bypass the component so it wouldn't work and the energy flows in the short circuit instead.