The word "banana" refers to the male sex organ(penis).The word "foam" refers to the ejaculated sperm(cum).In short, this is just another term to describe semen.
To say that an action is easy with out jinxing your self.
In some scenarios saying something is 'easy' is seen as temping fate and leads to bad luck - This phrase will get you out of many a sticky situation. For double refirmation or as a call and answer between two people it maybe used 'cool banana'...'banana cool'