To speak plainly - to describe something as it really is.
It might be thought that this derives from the derogatory slang use of the term spade meaning Negro - an American term originating in the 20th century. That view of it as derogatory might also be thought to be supported by this piece from John Trapp's Mellificium theologicum, or the marrow of many good authors, 1647:
"Gods people shall not spare to call a spade a spade, a niggard a niggard."
The phrase is much older than that though. Nicolas Udall, in his Apophthegmes, that is to saie, prompte saiynges. First gathered by Erasmus - translated 1542, has:
"Philippus aunswered, that the Macedonians wer feloes of no fyne witte in their termes but altogether grosse, clubbyshe, and rusticall, as they whiche had not the witte to calle a spade by any other name then a spade."
This refers back to Plutarch's Apophthegmata, 178 BC.
The eccentric right-wing British Tory politician Sir Gerald Nabarro was fond of emphasizing his direct 'man of the people' image by saying 'I call a spade a shovel'. In fact, despite being from an immigrant family himself, Nabarro loudly supported the repatriation of Caribbean immigrants to the UK. How he referred in private to the people who would have undoubtedly have been called 'spades' in Nabarro's social circle isn't recorded.