Menu toolbar:

start receiving images
stop receiving images
force a restart (e.g. when you see a wrong synchronization then click the restart button).
save the image at any time using the save button. You will probably want to use this if the Auto-save is not selected. Note only active in SSTV mode.
erase the screen

Sync Indicator (vu-meter S):

Volume (vu-meter V):

SSTV tab:

Use VIS Code: the reception will only start if a correct VIS code is received. If left unchecked, QSSTV will auto-detect the modes (this is the most used method).
Auto-slant: A lot of times the receive clock of the local station is not equal to the transmit clock of the sending station. The result is a slanted image. If The auto-slant is activated, the program will automatically try to compensate for those clock differences. You may notice a momentary freeze of the image build-up while the program is recalculating the image.
Auto-save: When the program detect the end of the image, the image will be saved to disk.
Save if Complete %: Auto-save if the image is at least completed for the indicated percentage.
Mode: you can either select AUTO or one of the supported SSTV modes. When AUTO is selected, the program will try to determine the mode of the image it is receiving. If a specific mode is selected the program will reject all images except if they are transmitted in the selected mode.
Sensitivity: Low, Normal, High and DX. When the Sensitivity is set to DX the signal will run until the end of the image before resynchronization.

Spectrum and Waterfall:
There are three markers for SSTV (red lines):

1200: Sync frequency
1500: Lower video frequency
2300: Upper video frequency In DRM, the markers indicate the 3 unmodulated carrier frequencies