Every step you click writes a line of M, Power Query's formula language. View > Advanced Editor shows the whole query:
let
Source = Csv.Document(...),
#"Promoted Headers" = Table.PromoteHeaders(Source),
#"Filtered Rows" = Table.SelectRows(#"Promoted Headers", each [Amount] > 100)
in
#"Filtered Rows"
A query is a let list of named steps, and in names the step that's the result. Each step reads the one before it by name. Names with spaces are written #"Like This". each starts a small function that runs once per row, and [Amount] is that row's Amount.
The formula bar shows the selected step. Edit it there and press Enter to change the step, such as a filter threshold. Rename a column, and later steps that still use the old name break with "The field ... of the record wasn't found".