NAND means NOT AND it complements the output of an AND gate. The symbol of NAND by a NOT gate. Generally Not operation is represented by a bubble as shown in figure (b) truth table of NAND gate is shown in figure.
If two inputs A and B are applied to NAND gate output (Y) gate output (Y) is the complement of the product of the inputs. When all the input are high the output is low . if any one or both the input are low then the output is high.
Figure (a) and (b) symbol and (c) truth table of a two input NAND gate