Imagine a nucleus (simply as a spher), with a much smaller electron bound to it. This means it will keep flying in close proximity to the nucleus. The distance between the nucleus and the electron will not be the same at all times. At one moment of time, the electron may be positioned a bit further away than average, or a bit closer than average. If we look at an s-electron, it will on average move relatively close to the nucleus, and sometimes even overlap with it. This last part means the s-electron can sometimes be found inside the nucleus.