What is soluble carbonate?

Hint: Carbonates are those compounds in which the anionic part of the ionic compound is carbonate ion that is written as $CO_{3}^{2-}$. Very metals form carbonates that are soluble in water and the atomic number of the metals are 11, 19, 37, etc.

Complete answer:
The term soluble means the given compound is miscible in the given solvent usually we consider the solvent like water. Carbonates are those compounds in which the anionic part of the ionic compound is a carbonate ion that is written as $CO_{3}^{2-}$. Some examples of carbonates are lithium carbonate ($L{{i}_{2}}C{{O}_{3}}$), magnesium carbonate ($MgC{{O}_{3}}$), Sodium carbonate ($N{{a}_{2}}C{{O}_{3}}$), potassium carbonate (${{K}_{2}}C{{O}_{3}}$), etc. Mostly all the carbonates are insoluble in water means when they are mixed into water, they remain as a separate layer at the bottom of the water.
But the carbonates of alkali metals especially, Sodium carbonate ($N{{a}_{2}}C{{O}_{3}}$), potassium carbonate (${{K}_{2}}C{{O}_{3}}$), rubidium carbonate ($R{{b}_{2}}C{{O}_{3}}$), and cesium carbonate ($C{{s}_{2}}C{{O}_{3}}$) are soluble in water which means when they are mixed in water they form miscible solution. The carbonate other than alkali metal soluble is ammonium carbonate. Its formula is ${{(N{{H}_{4}})}_{2}}C{{O}_{3}}$.
Not all the alkali carbonates are soluble in water, lithium carbonate whose formula is $L{{i}_{2}}C{{O}_{3}}$ is insoluble or little soluble in water and the carbonates of alkaline earth meats are also not soluble. Some bicarbonates are soluble in water.

Note:
Lithium carbonate is insoluble is due to the fact that lithium is the cation and its size is very small, but the size of the alkali metal cations are larger which makes them soluble in water.