These are basically digital versions (like ebooks) of worksheets that can be bought and downloaded to your computer from the internet or from CDs and then printed out. The advantage is that you can print out whichever sheet you wish. If you need to re-inforce a certain concept more you can usually print out that sheet again. Of course you will therefore need to have a computer and a printer.
If your child is used to doing a worksheet or two, they will be able to do this quite easily, and will understand that they need to complete the task before they can move on to the next activity. Worksheets also help your child to learn how to follow instructions, and teach them about following rules.
Almost everything you do with your child can be turned into an opportunity to learn - and worksheets can give you the guidance you need to find those opportunities.
This will include counting to ten, recognizing shapes and colors, being able to hold a pencil or crayon properly, and coloring in without scribbling. Basic math concepts such as recognizing patterns, understanding quantity and some simple addition and subtraction will be useful.