Good morning! In English the toe of a shoe or boot refers to that part of the shoe or boot where the person's toes are when they are wearing the shoe or boot. The toe is therefore the front part of a boot or shoe. The same applies to 'heel', in other words, one speaks about the heal of a shoe or boot when referring to the back, underpart of the boot or shoe, where the person's heel would go when wearing the boot or shoe. So the joke is that the person in question is too slow in their understanding to grasp that tipping the boot would allow the water to run out of the holes in the front (toe) of the boot (if the boot was full of water), even if the instructions for doing so were printed on the underpart of the back (heel) of the shoe. (It is actually a very unkind joke, making fun of someone's lack of intelligence, and probably not funny at all.) However, the important point is that some of the parts of a shoe correspond to the parts of a foot.

US - It is saying that someone is very, very stupid. Is it hard to pour water out of a boot? (no.) if you can't do that, you must be dumb. It is even easier to pour water out of a boot if there are holes in the toe. It is even *easier* than that, if there are instructions on the heel of a boot, because when you turned it over to read the instructions, the water would fall out. So, someone who can't do that is very, very, very stupid. Also, the expression is usually "couldn't pour piss out of a boot." (it's a bit vulgar).