What is a 2 to 4 line decoder?
A decoder takes in an address and then activates the output line corresponding to it. Pulling that line high or low depending on the decoder.


image source: wikipedia

The 2to4 means it takes a 2 bit address and controls 4 outputs. The number of outputs is always \$2^{inputs}\$.

They typically have an enable input to make it ignore the input and turn all outputs off. That way you can cascade them.