Circuit Diagram Conventions: wiring a MOSFET gate to "CE"?
It stands for "charge enable" (active low), and is an external signal to enable/disable charging.  If you don't want to disable charging you can omit the FET.

From the data sheet:

MPPSET:  Input voltage set point. Use a voltage divider from input source to GND to set voltage on MPPSET to 1.2 V. To disable charge, pull MPPSET below 75 mV.

So the FET gate can be attached to a logic level signal to pull MPPSET below 75mV disabling charge.