What are VDDA and VSSA?

VDDA and VSSA are the separate analog supply and analog ground pins for the MCU.

These pins are directly powering only the analog components such as ADC inside the MCU.

It differs from the digital power supply so that the digital supply pins power all the digital components in the MCU.

They are separated so the user can route the power pins separately and filter any digital noise so that the analog supply is clean and does not have much noise which could affect the ADC reading for example.

So it's not a better supply, the MCU needs all power supply and all ground pins to be connected. It just powers analog components from separate pins.

As the analog and digital parts must communicate between eachother inside the chip, the analog and digital supplies must be within range given in datasheet for the part to work. In this case, the VDD and AVDD must be connected to same voltage supply, as they are not allowed to differ more than 50mV when running, and up to 300mV during power-up and power-down transitions.

V<single letter>: The voltage associated with a particular thing, i.e. Ve is the emitter voltage of a BJT, Vs is the source voltage, etc.

V<single letter><different letter>: Usually more for theory, the voltage going from something to something, i.e. Vbe is the voltage from base to emitter on a BJT.

V<two same letters>: The supply voltage typically associated with a terminal on a transistor. I.e., VEE is the emitter (almost always negative) supply in a BJT circuit, VDD is the drain (almost always positive) supply in a MOSFET circuit (even though a CMOS circuit will have sources connected to either VDD or VSS, because the transistors are a mix of NMOS and PMOS).

V<two same letters><some other letter>: The supply voltage typically associated with a terminal on a transistor, only special somehow. In your case, VSSA means "lower voltage analog supply rail", and it's mated with VDDA, which is the higher voltage analog supply rail. So VSSD would be the digital VSS, VDDD would be the digital VDD (and I've never seen VDDD, but strange things are out there). It's certainly conceivable that you'll see VDD1, VDD2, etc., on a multi-supply board or chip.