I'm sure most of the fellows already expressed what is liquidity.

For the tips, I'd just like to advise the basic things as low liquidity pairs are unreliable so better not to take major trades in those pairs.

Use limit orders, in such pairs to avoid any uncertainty.
If you want to avoid liquidity-related issues better prefer a reputed exchange, most of the time in top-tier reputated exchanges liquidity-related issues never arise.