There is no description of node in syntax of TICKScript, because that is not syntactic concept, but semantic. Node is approximately Function { Chain }
in EBNF.
Here is description of nodes from documentation: https://docs.influxdata.com/kapacitor/v1.5/nodes/
Nodes represent process invocation units that either take data as a
batch or a point-by-point stream, and then alter the data, store the
data, or trigger some other activity based on changes in the data
(e.g., an alert).
You could see visualization of nodes, when you run kapacitor show <your_task_name>
, and run DOT section of output through dot
command. You will get something like:

On that picture nodes of graph will correspond to nodes of your TICKscript.