What is android:weightSum in android, and how does it work?

Adding on to superM's and Jeff's answer,

If there are 2 views in the LinearLayout, the first with a layout_weight of 1, the second with a layout_weight of 2 and no weightSum is specified, by default, the weightSum is calculated to be 3 (sum of the weights of the children) and the first view takes 1/3 of the space while the second takes 2/3.

However, if we were to specify the weightSum as 5, the first would take 1/5th of the space while the second would take 2/5th. So a total of 3/5th of the space would be occupied by the layout keeping the rest empty.

Per documentation, android:weightSum defines the maximum weight sum, and is calculated as the sum of the layout_weight of all the children if not specified explicitly.

Let's consider an example with a LinearLayout with horizontal orientation and 3 ImageViews inside it. Now we want these ImageViews always to take equal space. To acheive this, you can set the layout_weight of each ImageView to 1 and the weightSum will be calculated to be equal to 3 as shown in the comment.

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    <!-- android:weightSum="3" -->
    android:orientation="horizontal"
    android:layout_gravity="center">

   <ImageView
       android:layout_height="wrap_content"       
       android:layout_weight="1"
       android:layout_width="0dp"/>
  .....

weightSum is useful for having the layout rendered correctly for any device, which will not happen if you set width and height directly.

Weight sum works exactly as you want (like other answers you don't have to sum all the weights on parent layout). On child view specify the weight you want it to take. Don't forget to specify

android:layout_width="0dp" 

Following is an example

    <LinearLayout
                android:layout_width="500dp"
                android:layout_height="20dp" >

                <TextView
                    android:layout_width="0dp"
                    android:layout_height="match_parent"
                    android:layout_weight="3"
                    android:background="@android:color/holo_green_light"
                    android:gravity="center"
                    android:text="30%"
                    android:textColor="@android:color/white" >
                </TextView>

                <TextView
                    android:layout_width="0dp"
                    android:layout_height="match_parent"
                    android:layout_weight="2"
                    android:background="@android:color/holo_blue_bright"
                    android:gravity="center"
                    android:text="20%"
                    android:textColor="@android:color/white" >
                </TextView>

                <TextView
                    android:layout_width="0dp"
                    android:layout_height="match_parent"
                    android:layout_weight="5"
                    android:background="@android:color/holo_orange_dark"
                    android:gravity="center"
                    android:text="50%"
                    android:textColor="@android:color/white" >
                </TextView>
 </LinearLayout>

This will look like