Complex sentence: For the same asset, its liquidity can change through time or between countries, based on the market liquidity for the asset at the time or in the country, etc. The liquidity of a product can be measured as how often it is bought and sold.
Split into simpler sentences: For the same asset, its liquidity can change through time or between different markets, such as in different countries. The change in the asset's liquidity is just based on the market liquidity for the asset at the particular time or in the particular country, etc. The liquidity of a product can be measured as how often it is bought and sold.