In Lean PlanDo, critical path is one of the most important features to allow the user to track which works are the most important and critical to the project. These works are called critical activities, and the chain of these critical activities make up the critical path. 

Basically, these are the activities that are having 0 floats in the schedule. Float value is the number of days that an activity can be delayed, without affecting its successor. For example, Activity A is the predecessor of B with FS0 dependency. If B is currently scheduled 3 days after the end date of A, that means A has 3 days float. A can be delayed or extended 3 more days until it affects the start date of B. When the activity has no project float, any change to it will also affect the project. This float value is shown in the widget and detailed view of each activity.

In the gantt chart, critical activities are represented by light red bars. If you check all the activities with this color, they will be having 0 floats.

In the Masterplan, you can easily filter out the critical activities or critical path. Click the filter button and check the "Critical Works" checkbox, then click Apply. The masterplan will now only show the critical path.