The UWL emphasizes (bold) items based on the view configuration.
In the default view, items are emphasized, i.e. shown in bold style if they are new. As soon as a given item is processed by the user, its status will change from "New" to "In process" and the item will no longer be considered as new and will not appear bold.
The default behavior is configurable. For more information see the documentation about the DTD Configuration. In the SAP NetWeaver Technology Consultant's Guide for Business Task Management, choose Universal Worklist Configuration > Configuration DTD > View.