I am new to sheet music. I am trying to recreate in Guitar Pro, corresponding to guitar strumming, the music sheet notation of the picture below. The timing is 4/4 and tempo is 120. I am unable to create or understand the meaning of a top and bottom beam together.
The top beam, attached to all of the Ds, indicates that those notes constitute one "voice". The notes (chords) attached to the bottom beams are additional "voices".
Imagine instead of guitar, this were a work for choir with soloist. The top "voice" would be sung by the soloist, and the bottom "voices" would be sung by the different parts of the choir.
All the vertically-aligned notes are still played simultaneously. The way the separate beams affect the playing of the passage is that the "lead singer" voice — the Ds — should be played a bit louder than the "backup singer" voices.
You can access and enable the Multivoice feature in Guitar Pro 7 from the Edition Palet located on the left of your window, or even access it from the Edit > Voices menu. (Guitar Pro 7.5 support page)
How it should look when done properly
(Note: The notation is correct, but I didn't set things up to make the tablature match the OP version.)
This second version is also acceptable and, IMO, easier to read: