"If it is intentional, it could be called deceit, lying, misleading, mendacity, etc etc.
Otherwise it could be a slip of the tongue (parapraxis) where a word is accidentally replaced by another. In a sexual (or otherwise nervous) setting, a slip of the tongue can be called a Freudian Slip. eg saying "I have to wash his balls" instead of "I have to wash his smalls"
It can also be innuendo, whereby ones says something which is apparently innocent but means something else, usually of a sexual nature eg What does it mean if someone says to a pretty market stallholder "Can I feel your melons?"
It can be metaphor, where a difficult idea is expressed in simple or picturesque terms eg "It's raining cats and dogs" to mean "Its raining heavily".
It can be euphemism, where the word(s) used substitute for other words or ideas that are being avoided, for reasons of sensitivity, secrecy, etc.
It can be hyperbole, (exaggeration) where the truth is stretched for emphasis eg "I've told you a million times not to do that" when you mean "I've told you many times"
It can be metonym, where a simple idea is used to represent a larger concept eg the White House to represent the US presidency. You might say "The White House is debating the war" when you mean "The Presidency is discussing the war".
It can be slang. A Londoner might say "Where's my trouble and strife" when he means "Where's my wife"
It can be a malapropism where a word is accidentally replaced by a similar sounding one eg "I can say without fear of contraception.." instead of "I can say without fear of contradiction.."
It can be a spoonerism, where the initial letters of two words are swapped eg saying "Its roaring with pain" instead of "Its pouring with rain""