Hello! Thermal equilibrium is reached between two objects when both have the same final heat value or temperature. Meaning when ice is dropped into water, and the ice melts, the ice and water mixture will get to thermal equilbrum when the temp of the water has stabilized and the ice added has completely melted and warmed up to the water temp so ice and water are now in thermal equilibrium.
Hope this helps you understand the concept. Any questions please just ask. Thank you!!