What does this mean?

The power GaAs FET is internally matched for
standard communication bands to provide optimum power and gain in
a 50 ohm system.
What is "internally matched"?

internally= inside the FET case there is the bare chip and an input or output or both matching networks.
since the network(s) are placed very close to tha bare chip, they are very efficient and the PowerFET became an PA.
In Practice to get higer efficiency an additional exernal, network is often required.

In addition to the above, these internal networks are very narrow band and such transistors are made for specific markets with specific, narrow frequency ranges.

This is a disadvantage if yo want to use them for some other frequency range. Ordinary transistors without internal matching can be used for a wide range of frequencies because you can design the external networks.