Buy and sell side liquidity are areas of price in which buy stops and or sell stops are mostly residing. if you can understand the higher time frame perspectives and see where the "money" is, then you have a bias once you see price moving off known areas of support or resistance. Price will seek the liquidity in order to either reverse or continue in within it's expansion move. If we see price as reversing at the buy or sell side of liquidity, then we trade the developed price action, if we see price continue to move through the buy or sell side liquidity, then we stick to the order flow of price and trade the buy or sell side that it is, towards the next price objectives.