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As each word means, head of streamblock is "N" amount of frames at the beginning of a streamblock, and tail of streamblock is "N" amount of frames at the very end of a streamblock.

i.e. Considering a streamblock sending 100 frames, and being "N" = 20:
 

Usually those terms are meaningful when in terms of drop frames, because it is well known that the advanced Sequence algorithm to determine how many frames have been dropped knows only what has been received, so when "N" amount of the first frames (head of streamblock) from a stream are NOT received on a port, the Dropped Count (Frames) value on the Advanced Sequence tab will be zero.

The same happens for example, If the first 100 frames from a stream arrive on a port, but the stream is entirely dropped after that, we can say the tail of streamblock has been dropped, however at this point the Dropped Count (Frames) value will also be zero. If another frame arrives from that stream, the Advanced Sequencer immediately calculates how many frames have been dropped in between.