A small gap pattern around a pad inside a copper pour. Lets you solder the pad without the whole pour sucking heat away. KiCad does this automatically when pad_connection: thermal.
How it looks
Drawing: this is not on CORE3588, so it is drawn
Amber is a ground copper pour; the light rectangle is a pad on the same net. Left: joined solid. Right: joined through four spokes across a gap.
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
CORE3588 does not use thermal reliefs. Its ground pours are set to join pads solid (all its parts are soldered in a reflow oven by the assembler). A 3.2 x 3.2 mm patch of top copper.
1Ground pad of capacitor C3 (amber), fused straight into the grey ground pour: no gap, no spokes. Its other pad, on a different net, has the usual clearance gap.
How it works
Where it fits
A setting of a copper pour, applied to the pads it swallows. It matters most for hand soldering and for heavy ground planes; reflow heats the whole board more evenly.