neural network - target data
The target data is the desired output that happens to be in the real word. This information is known to you already(given the problem is supervised learning problem). So, for an 'AND' gate the target value is the vector stored in y on the right hand side as shown below:

x1    x2      y(desired output)
0     0          0
0     1          0
1     0          0
1     1          1


So this vector y = [0,0,0,1] is your target to these input types that you feed into the network as labels corresponding to your above input data.