The Percentage Split settings field is where you can assign the percentage of contacts that will be sent down that particular path in the workflow. If the Percentage Split element is only connected to one other element, you can only set the percentage to 0% or 100%. However, if the element is connected to multiple elements, you may adjust the percentage of each element as you like by entering a number in the Weight field.
When a Weight number is assigned to each connection, the system will automatically calculate the difference between those numbers in percentages with the total being 100% between them all. For example, if you connect the Percentage Split element to two other elements and give each connection a Weight of 1, the split between the two will be 50/50 meaning there is a 50% chance of contacts going down each path.
However, if you give the first connection a weight for 1 and the second weight of 2, the chances of contacts going down the second path should be about double that of the first connection. So, in this case, contacts will have a 33% chance of going down the first path.