From IBM research blog under the subtitle "Resetting a qubit":

Internally, these reset instructions are composed of a mid-circuit measurement followed by an x-gate conditioned on the outcome of the measurement.

According to the paper "A Thorough Study of State Leakage Mitigation in Quantum Computing with One-Time Pad" by Xu et la,^[1]

Thermal Relaxation: This reset approach simply idles the qubit for a long time to allow decoherence to occur. Currently, quantum computers on most cloud platforms, such as IBM Quantum, are mainly using this approach.

reset instruction: The typical implementation of a “reset instruction” is based on a mid-circuit measurement followed by a conditional Pauli-X[...] IBM Quantum adopts this implementation.

So, thermal relaxation is used as a reset mechanism before running a new circuit, and measurement followed by a conditional-X is used for mid-circuit reset operations.