This is pretty dull. Click Start and you'll see our lone cell dies because it has fewer than two live neighbours.
This is also pretty dull. Click Start and you'll see both cells dies because each has fewer than two live neighbours.
Three Cells - The Blinker
This is... interesting? Click Start and you'll that the pattern oscillates.
On the first generation:
The centre cell remains alive because any cell with two or three neighbours lives.
The top and bottom cells both die because any cell with fewer than two live neighbours dies.
The left and right cells are both born because any dead cell with exactly three neighbours becomes a live cell.
This is inverted on the second generation and again on the third and so on..
The Life world has its own rich lexicon. An oscillator is a pattern that repeats itself after a fixed number of generations (this is the period). This particular oscillator is called a Blinker and has a period of 2.