Yes the Roku channel itself is free to watch.
But there are some paid shows and programs on the Roku channel but you don't have to pay for the channels if you do not want to watch them.
You can just watch the programs and shows that you want to watch for free without paying a subscription fee for the Roku channel.
Roku can be used to watch many other channels on the Roku device without a subscription fee but some other channels on the Roku device such as Netflix or Hulu require a monthly subscription fee to watch.
But you don't have to pay for the channels if you don't want to watch them.
I don't pay any subscription fees to watch channels on my Roku Device because the free channels are good enough for me as I do not watch all that much television.