prefixing text with a colon gives a Symbol, e.g. :foo, :bar. The Dataframes package allows a DataFrame to be indexed with a Symbol to refer to the column with that name.
The PyCall package allows python libraries to be called from Julia code, but translates a obj.field in Python into obj[:field] in Julia. see the usage section of the docs for details.
also - please surround quoted code with back-ticks so it gets nice syntax highlighting and is displayed in a fixed-width font. Single backticks are for in-line code like this, and triple quotes are used for longer code:
like this