Dimension of 1x1 convolution output
Have a look at this:
https://medium.com/machine-learning-bites/deeplearning-series-convolutional-neural-networks-a9c2f2ee1524

It's like this:
third dimension of input and filter must match!
So if you only have one of those yellow filters your output will be 6x6x1. But if you have let's say 42 filters then you'll perform 42 filter operations and your output will be 6x6x42.

Hope this helps ;)