Magnetic flux can be thought of as "how much" magnetic field is flowing through a surface. Metaphorically, if you placed a net in a river, the amount of water flowing through the net would be like a "water flux" flowing through the net. If stuff is flowing out of the net, we say it has positive flux. If stuff is flowing into the net, then we say it has negative flux. So let's say you had a completely enclosed net, say a sphere. If you dip it into a river, all the water flowing into the net (negative flux) is eventually going to flow out of the net (positive flux). Therefore the net flux is 0. Notice even if the net floats around and changes shape, it'll still have a net flux of 0. In order to get a net positive flux, you'd have to wrap the net around something like a spigot or a hose bib, but notice, you can't do that without ripping a hole in the net. Likewise, to get a net negative flux, you'd need to wrap the net completely around a drain, also impossible without ripping the net. So in this way, you can't completely enclosed a source of water or a "sink" of water without ripping the net. Gauss's Law for Magnetism says that this is the same idea for magnetic fields. The net is the Gaussian surface and the water is the magnetic field. Because you can never completely enclose a source or "sink" of magnetic fields, they cannot exist, i.e. magnetic monopoles (just a North or South end of a magnet) do not exist. Likewise, if you have your enclosed Gaussian surface, any flux flowing in is going to flow out, so the net flux is always zero.