A thermal relief pad is a printed cicuit board

Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

 (PCB) pad connected to a copper pour

Copper pour

In electronics, the term "copper pour" refers to an area on a printed circuit board filled with copper . Copper pour is commonly used to create a ground plane...

 using a thermal connection. It looks like a normal pad with copper "spokes" connecting it to the surrounding copper.

A typical pad on a printed circuit board is only connected to a few narrow tracks at most. A pad directly connected to the copper pour would be difficult to solder

Soldering

Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...

, since the heat provided by the soldering iron

Soldering iron

A soldering iron is a hand tool most commonly used in soldering. It supplies heat to melt the solder so that it can flow into the joint between two workpieces.A soldering iron is composed of a heated metal tip and an insulated handle...

 will quickly leak away from the pad and into the copper pour (due to high thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

 of copper). A thermal connection restricts the heat flow, making the pad easier to solder.
Via holes that connect tracks on one layer to another or between tracks and planes do not need thermal relief, unless soldering wires or pins into the hole.
Wire leaded components that are either carrying Radio Frequency

Radio frequency

Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 currents, where the additional inductance

Inductance

In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

 would be problematic, or where very high current densitities are expected, and the spokes of the thermal relief may act as a fuse (electrical)

Fuse (electrical)

In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...

, may require the thermal relief pattern to be customised or even ommitted, in which case the parts may require additional hand soldering during assembly.