What's the meaning of Web Page life-cycle?

The page lifecycle is the sequence of events that are invoked in an ASP.NET Page Request.

This is documented in great detail here.

As you tagged your question with ASP.NET, here goes the mandatory doc:

ASP.NET Page Life Cycle Overview

When an ASP.NET page runs, the page goes through a life cycle (stages) in which it performs a series of processing steps. These include initialization, instantiating controls, restoring and maintaining state, running event handler code, and rendering. It is important for you to understand the page life cycle so that you can write code at the appropriate life-cycle stage for the effect you intend.