‘And he says to them, “Are you also so without understanding? Do you not perceive that whatever from without goes into a man cannot defile him, because it does not go into his heart, but into his stomach, and goes out into the latrine, purifying all the food.” '
It is clear from this that Jesus was beginning to expect more of His disciples and was a little disappointed at their slowness. The point was that only what affected the heart of a man, the essential man, could really defile him spiritually. But food merely goes into the stomach, and the resulting waste then goes into the latrine (toilet, cesspit). So if we see the final words as spoken by Jesus He is saying that it is clear from everyday experience that in this way the food has been purified and any defilement removed, for it comes out at the other end. That makes it clear that that it is not what he eats or how he eats that makes him sinful, it is what he allows to come from within his heart.
Some, however, see ‘purifying all the food' as a comment added by Mark signifying that the assumption must therefore be that all meats are clean. RV translates it as, ‘this He said making all meats clean', but ‘this He said' is not in the text, it is read in to make the sense and may be wrong. The principle is, however, correct. Essentially in what He was saying Jesus was saying that food cannot defile a man.