Fractional shares support means that the broker allows you to buy or sell a fraction of a stock - for example only 0.02 stock.
Its availability depends on the broker and the symbol - it is up to broker discretion if it supports fractional shares on stocks and on which symbols exactly.
Please check the updated documentation for your broker, at the time of writing this article the situation is as follows.
InteractiveBrokers
Documentation link: https://www.interactivebrokers.com/en/trading/fractional-trading.php
supports all order types
not supported on demo and through API
standard commission fee applied List of supported stocks (regularly updated): http://www.ibkr.com/download/fracshare_stk.csv
Alpaca
supports Market and day order types
Not all assets are fractionable
XTB
Documentation (in Czech): https://cz.xtb.com/frakcni-akcie
there is a column Fractions Yes/No in the instruments table: https://xtb.scdn5.secure.raxcdn.com/file/0094/51/newomitablecz24042023_f5dc304de0.pdf