Black holes are so massive that they severely warp the fabric of spacetime (the three spatial dimensions and time combined in a four-dimensional continuum). For this reason, an observer inside a black hole experiences the passage of time much differently than an outside observer. Imagine you want to investigate a black hole by shining a light towards it and measuring the time that elapses before the light is reflected back to you. Unfortunately, you will be waiting a very long time—forever, in fact. The light will appear to continually slow down as it approaches the black hole, ultimately reaching a complete dead stop at the event horizon.
Now imagine your colleague Sally is interested in more hands-on investigation of time inside a black hole, and decides to dive towards it. Before she leaves, Sally agrees to flash a light back to you every second. From your perspective, Sally appears to slow down as she approaches the black hole, and the time interval between her flashes of light gradually increases. Additionally, the light she sends back to you gradually gets dimmer and redder.
According to your perspective, Sally never actually descends into the black hole; she will travel more and more slowly as she approaches the event horizon, but you will never actually see her reach “the point of no return.” Time comes to a standstill at the event horizon, such that an outside observer will never really see anything fall inside a black hole. Strangely enough, this even includes the surface of the star that collapsed to form the black hole!