What is meant by abstract="true" in spring?

Abstract beans in Spring are somewhat different from abstract classes. In fact, abstract bean in Spring doesn't even have to be mapped to any class. Take this as an example:

<bean id="dao" abstract="true">
    <property name="dataSource" ref="dataSource"/>
    <property name="someHelper" ref="someHelper"/>
</bean>

<bean id="fooDao" class="FooDao" parent="dao">
    <property name="fooHelper" ref="fooHelper"/>
</bean>
<bean id="barDao" class="BarDao" parent="dao">
    <property name="barHelper" ref="barHelper"/>
</bean>

And classes:

public class FooDao {
    private DataSource dataSource;
    private SomeHelper someHelper;
    private FooHelper fooHelper;

    //setters
}

public class BarDao {
    private DataSource dataSource;
    private SomeHelper someHelper;
    private BarHelper barHelper;

    //setters
}

Note that FooDao and BarDao do not have any parent (abstract or not) base class in common. Parent abstract bean definition is used only to group common properties, so you avoid repetition in XML.

On the other hand introducing abstract Dao class that both FooDao and BarDao inherit from would be a good idea:

public abstract Dao {
    protected DataSource dataSource;
    protected SomeHelper someHelper;

    //setters
}

public class FooDao extends Dao {
    private FooHelper fooHelper;

    //setters
}

public class BarDao extends Dao {
    private BarHelper barHelper;

    //setters
}

But still dao bean doesn't have to define a class. Treat abstract beans as a way to reduce duplication in XML when several concrete beans have same dependencies.

Actually, abstract parent bean is not necessary to define class attribute, you may just need a common property for sharing.

According to this tutorial

The parent bean cannot be instantiated on its own because it is incomplete, and it is also explicitly marked as abstract. When a definition is abstract like this, it is usable only as a pure template bean definition that serves as a parent definition for child definitions.