What does the term "stackable switches" means?

Stackable switches are a type of switch that is designed to be stacked on top of one another. Instead of the rack-mounted switches, these switches tend to have the ability to be placed in networking closets and stand alone. The feature sets of these stackable switches vary depending on vendor and platform. Many of the stackable switches do support advanced functionalities like QoS, multicasting, and VLAN management. PureData and DLink both have stackable switches that support QoS policing in some form or fashion. Refer to the preferred vendor support matrix for the features you require.

For access layer switching technologies, choose the switches that support VLANs, STP, and IGMP. QoS support is an added bonus in this scenario but not explicitly required. I highly recommend the application of QoS at the aggregation points for your switching architecture, i.e. where all of your switches start to feed into the core. This is where QoS, scheduling and queuing for the most part, is essential.