Gravity, as described by Einstein's equations of general relativity, predicts that light waves follow a path that can be curved by a gravitational field. A similar phenomenon is seen in waves on the surface of a body of water. In the case of water waves, the wave path is curved by the contour of the bed underlying the water. In locations where the water is deep, the wave speed increases. In places where the water is shallow, waves move more slowly. Thus, a wavefront can be curved as the wave passes an island, or a shoreline, where the depth of the water is changing.
In the first movie, the results of a simulation of water waves moving over a submerged conical island are presented. Wavefronts are generated and the waves cross the submerged island and become curved. Once the wavefronts pass the island, the waves are no longer moving parallel to each other. Their paths cross. When the wavefronts cross, an interference pattern is created behind the submerged island.
In the second movie are results of a simulation in which the submerged island has a parabolic profile. This causes the wavepaths to get so bent that they actually circle around the island. This is similar to what can happen around a black hole. Around a black hole, spacetime is curved enough that light wavefronts become so bent that they can never escape from the black hole.
In the third movie, a different situation is presented. What you see is a cosmic string passing through a gas of particles. The movie is actually a slice through a three-dimensional volume, so the cosmic string, which should be long and thin, looks like a point. That's because we're just looking at the matter in a slice and the string goes through the slice at a point. The first thing you need to know is that the spacetime around the cosmic string is curved and causes particles to be deflected in much the same way that waves are deflected as they pass the submerged conical island.
To understand the effect of the string passing through the gas, think what would happen if the string were standing still, but the gas were passing by it. The particles in the gas would follow similar trajectories to that of the waves passing the submerged conical island. Their trajectories would cross behind the island. Now, if the cosmic string is moving through the gas of particles, the same thing happens, particle trajectories cross behind the cosmic string's path and clump up in what's called a wake. The difference is that once the particles form a wake behind the string, the particles in the wake have their own gravitational field and the wake accretes (pulls in) more of the gas, so the wake caused by the string grows.
The perturbations caused by cosmic strings have are the size of many galaxies and, if cosmic strings exist (and there are reasons to believe they should) are thought to be the places where galaxies and stars form.