Firstly, welcome to ASF!
What you are referring to is liquidity. In simple terms - how easy it is to actually find a buyer or seller.
For large cap companies with a very active market in their shares, there's no real issue since a huge number of trades take place. For example, looking at the 3 largest companies listed on the ASX and considering today's trades only:
CSL Limited (CSL) - 1,108,085 shares traded with 59,016 trades taking place.
Commonwealth Bank (CBA) - 4,976,671 shares traded with 58,562 trades
BHP Group (BHP) - 11,522,505 shares traded with 82,953 trades
Those are the 3 highest valued companies listed on the ASX and as you can see, there's a huge volume of shares being traded so no real problem.
At the other extreme, Sagalio Energy (SAN) has a market capitalisation of merely $410K, that's the entire value of all shares in the company at their current market price, and literally zero were traded today. There are zero shares currently for sale at the price of the last trade and for less than $11,000 you could buy literally everything that's currently for sale - with the not insignificant problem that you'd be paying 50 - 100% above the last traded price in order to buy anything at all.
Making even a small investment in SAN would thus be difficult since there are few buyers, there's only 1 at the current market price and 1 other at literally half that amount, and there are few sellers - there's 1 at 50% above the last price and 1 more at double the last price.
In contrast, you could buy pretty much any volume of shares in BHP without even the slightest difficulty so long as you're willing to pay the market price. 82,953 trades today alone, so that's almost 4 trades every second when the market's open, and the total value of BHP shares traded just today was $388 million.
Many investors avoid investing in any stock which has low trading volumes due to the above problems. In doing that they'll be looking at average volumes and few or preferably no days with zero trades taking place.
Anything within the top 200 companies as a general rule won't have any issues like that, it's the smaller companies where it's a more common problem.