What is this?
Each visualisation contains some exported part of the Tangle history to graphically visualise how huge and beautiful the Tangle actually is.
To issue a transaction (dots in the visualizations), users must approve (lines in the visualizations) two other transactions. As a transaction receives additional approvals, it is accepted by the system with a higher level of confidence.
To get a more visual sense how does that work, visit - http://iota.dance/live/ for a live Tangle visualisation.