Above is a number line which is marked between -1 and 3, so the whole numbers on this line are labelled -1, 0, 1, 2, 3.
If you look at the numbers between 2 and 3 you can see 2.8. The space between each of the whole numbers is divided into ten equal parts (tenths), but the picture shows only 5 of these (fifths) to save space. For instance, 2.1 would be halfway between 2 and 2.2; 2.3 would be halfway between 2.2 and 2.4, and so on. So 2.8 is 8 tenths the distance between 2 and 3; or you could say that 2.8 is 4 fifths (it's 4 divisions to the right of 2) between 2 and 3: 2.2 is one fifth between 2 and 3 (1 division to the right of 2); 2.4 is two fifths between them; 2.6 is three fifths between them; 2.8 is four fifths between them. You can imagine the whole numbers as tall posts in the ground (represented by the number line) with 4 shorter posts between two consecutive tall posts. The spacing is the same between each of the posts between any two whole numbers. The number line picture helps you to understand how whole numbers and decimal fractions are arranged.
What about -2.8? How is it represented? Look at the number line between -1 and 0. Note that that's divided into 5ths, too, but the numbering is reversed because we count to the left of zero when we're dealing with negative numbers. So -0.2 is one fifth to the LEFT of zero and -0.8 is four fifths to the LEFT of zero. This pattern is repeated as we go more negative, so -1.2 is one fifth to the left of -1; -1.8 is four fifths to the left of -1; and -2.8 is four fifths to the left of -2. The picture doesn't show -2.8 but you should now be able to draw what it looks like by continuing the pattern leftwards up to -3 similar to the way the picture shows where 2.8 (positive) is. Remember that everything is related to where zero is. The number line is measuring how far to the right (positive) of zero we are, and how far to the left (negative) of zero we are. You can imagine your house being where zero is. All the houses to the right of yours (up the road) represent positive numbers while all those to the left (down the road) represent negative numbers. It's the distance of each of the other houses from yours that matters, and that's how you can think of positive and negative numbers. For instance, 3 houses up the road represent 3 (or +3, positive) while 3 houses down the road represent -3 (negative).
