I will go out on a limb here and "assume" that the poster is refering to "logic gate". This link offers detail of Logic gate: http://en.wikipedia.org/wiki/Logic_gate
Basically lets say you are using 2 normally open pushbuttons connected, in series, to a PLC. IF PB1 AND PB2 are pushed then you will have a signal present at the PLC. This is an example of an AND gate.
Another example is 2 normally opened pushbuttons connected in parallel, if PB1 OR PB2 is pushed then a signal is present at the PLC. This is an OR gate.