Gambeson:

This was the most common undergarment, made of layered fabric like linen or wool.
It provided padding to absorb the impact of blows and protected the body from the metal armor itself.
For poorer soldiers, a thick gambeson could also serve as their primary armour.

Other garments:

Shirt:
A simple linen or wool shirt was worn directly against the skin to wick away sweat and prevent chafing from both the padding and the armour.

Coif:
A padded or mail hood was worn under the helmet to protect the head and neck.

Mail shirt:
For those who could afford it, a mail shirt might be worn between the padded gambeson and plate armour for additional protection.