What does it mean when we say "a blackbody is in thermal equilibrium with it's surroundings"? Quoting from my text book - "a blackbody is a perfect radiator of radiant energy. It is in thermal equilibrium with it's surroundings and radiated as much energy per unit area as it absorbs from it's surroundings at any given time"
Is a blackbody always in thermal equilibrium with it's surrounding temperature? What is the meaning of "radiant energy"? Why should a blackbody be in thermal equilibrium with it's surroundings?
 A: 
Is a blackbody always in thermal equilibrium with it's surrounding temperature?

Any body would be in thermal equilibrium with surroundings only if their temperatures are same. If the black body was at different temperature, then it would radiate/absorb heat to gain the same temperature and this will take some time.

What is the meaning of "radiant energy"?

Every body radiates some energy via Electromagnetic Radiations and this is called here as "radiant energy".

Why should a blackbody be in thermal equilibrium with it's surroundings?

Forget about black body and take any body. This body will have a natural tendency to try and maintain a thermal equilibrium with its neighbours. If it were not so, that is, the temperature of the body was different from its surroundings, then in a net exchange with its surroundings, the body will radiate/absorb heat energy from its surrounding to get same temperature . This fact is both logical and experimentally verified.
More experimentally, when the body is at different temperature,it radiates more/less heat than it gains from the surroundings and hence its temperature changes. This net exchange is given by Stefan Boltzmann Law.
$$\Delta E_{body} = eA\sigma (T^4_{body}- T^4_{surroundings})$$
