Kdenlive uses 2 monitor widgets to display your videos: Clip Monitor and Project Monitor. A third monitor - the Record Monitor - previews video capture.
1) is Set In Point button - click this to set an 'in' point.
2) is Set Out Point button - click this to set an 'out' point.
3) Green selected zone - selected by setting in and out points. Draging the clip from the clip monitor to the time line when there is a green selected zone causes the selected zone to be copied to the timeline.
4) Position Carat - can be dragged in the clip. (In ver >=0.9.4 and with Open GL turned on in Settings -> Configure Kdenlive -> Playback audio will play as you drag this.)
5) Timecode widget - type a timecode here and hit Enter to go an exact location in the clip.
6) Timecode arrows - can use these to change the current position of the clip in the clip monitor.
Zones are defined regions of clips that are indictated by green in the clip monitors time line - see item 3 above. The beginning of a zone is set by clicking [ (item 1 in the pic above). The end of a zone is set by clicking ] (item 2 in the pic above)
The clip monitor has a right click (context menu) as described here
1) is the position Carat and shows the current location in the project relative to the whole project. You can click and drag this to move the position in the project.
2) is the timecode widget. You can type a time code in this and press Enter to bring the project monitor to an exact location
3) is the timecode widget control arrows. You can move the project monitor one frame at a time with these.
You can use the [ and ] buttons to create a zone in the project monitor in the same way as you do to make zones in the clip monitor. The zone will be indicated by a green bar in both the time line and underneath the project monitor.
You can get Kdenlive to only render the selected zone - see Rendering Using the Selected Zone Option
The project monitor has a right click (context menu) as described here
There is also a Record monitor that can be used to preview capture from Firewire, Blackmagic cards or through FFmpeg / video4linux