"what is an 'active low pulse'?"

Exactly what it says on the tin - a pulse that is active whilst it is in the low state.

Thus:
* an active-low 'Chip-Select' selects the chip whenever the signal is low;

* an active-low 'Write' signal selects the 'write' function whenever the signal is low;

* an active-low 'Read' signal selects the 'read' function whenever the signal is low;

etc, etc,...

This is, of course, the opposite of an active-high signal - where the function is active whenever the signal is high!