Silicone Pads, also sometimes referred to as Gino Pads.
These pads are 0.5mm thick silicone and are used to keep a glass printing surface connected to the heatbed (for optimal heat conduction). These pads does not use glue, so it does not make a sticky mess, as it simply sucks the glass to the heatbed.
This suction is strong enough to prevent the glass from sliding of on moving platforms (like the i3 Prusa designs), while also allowing the heatbed and glass to expand at their own rates. This prevents the glass or heatbed from warping due to the expansion of the other.
These pads come in blocks of about 30mm squares (although they are not always squares).
Placing 5 of these for a small heatbed (200×200) or 8 for larger heatbeds should be good enough.