Our brain has limited capacity of processing information due to its limited cognitive resources. We can focus fully on only one thing at a time, but we are able to switch our attention between tasks, allowing us to handle two or more activities in sequence, even if not perfectly simultaneously.

• Selective attention is the mental process of prioritizing certain inputs over others. It helps us focus on what is most important to us when there are many demands on our attention. However, if we do not consciously decide what to pay attention to, we may unintentionally ignore potentially critical information.
• Divided attention is the ability to pay attention to two or more tasks at the same time. Unlike selective attention, where you focus on one thing and ignore others, divided attention requires splitting your mental resources between multiple activities.

The pilot missing the glide slope while monitoring the speed. → Selective attention
The pilot switches his/her attention between the glide slope and the speed. → Divided attention