What is depth of field button in DSLR and its usage?

Question

Im a beginner.plz help.

Answer 1

The depth of field refers to how many objects are within different ranges.  Say, your friend is next to the tree.  Then she walks to you.  Do you focus on her or the tree?  

It can explain better in your book.  I haven't used my 35MM in ages since I got the digital.

Answer 2

When you keep it pressed in, the aperture blades close down to the selected/ computed value for the shot, and you can preview how much will be in focus.

With digital SLRs I've never found a good use for this button - you can also just take a test shot and check your LCD screen. This way, as a bonus, you can also check the histogram to see if you should adjust the exposure.