1. What is thermal equilibrium ?
A body is said to be in thermal equilibrium if thermodynamical variables like Pressure (P),Volume (V), and temperature (T) associated with body do not change with time.

2. Explain Zeroth’s law.
According to Zeroth’s law “If two systems ‘A’ and ‘B’ are separately in thermal equilibrium with a third system ‘C’ ,then the two systems ‘A’ and ‘B’ are also in thermal equilibrium with each other”.

Theory :

Consider two systems A and B seprated from each other by an adiabatic wall(one that conducts no heat) but each being in contact with a third system ‘C’ through a diathermic wall which permits heat to pass, then the system A will be in thermal equilibrium with C.
Now If adiabatic wall between systems A and  ‘B’ is replaced by a diathermic wall, experiments shows that no further changes occur in systems A and ‘B’ indicating thereby that system A was also in thermal equilibrium with system B. This important experimental fact leads us to following general conclusion :

“If two systems are in thermal equilibrium with a third system ,then they must be in thermal equilibrium with each other”.
This statement is called Zeroth’s law of thermodynamics.

Illustration :

A simple illustration of Zeroth’s law is provided in case of two gases A and  ‘B’ enclosed in a vessel and a mercury thermometer C .
If the there is no change in length of mercury thread on placing the thermometer C either in contact of gas vessel containing A or that of vessel containing B are brought in thermal contact with each other. This is in accordance of Zeroth’s law.

Zeroth’s law & Temperature :
Zeroth’s law forms the basis of concept of temperature. All these three systems can be said to possess a property that ensure their being in thermal equilibrium with one another.This property is known as temperature.
We may therefore define the temperature of a system as the property that determines “whether or not the system is in thermal equilibrium with neighbouring system”.