What is the low side on 1991 Nissan Pickup (D21)

what is the low side

The opposite of "the high side!"

The fitting to add more freon to the A/C system. I assume you have ask where to charge the A/C system and was told that. Instead of me typing a bunch of words, try youtube and search for a video on how to charge an A/C system. Bet you will find one!