A tune, actually.

[shootingstar]

. . .
'Nora, what's with you? Why are you strange today?'
She took off her shoes. 'I'm not.'
A memory came to her, related to the Jaws T-shirt.
A tune, actually. 'Beautiful Sky'.
The day she had bought Dan the Jaws T-shirt had been the day she had played him a song she had written for The Labyrinths.
. . .
(The Midnight Library by Matt Haig, episode The Three Horseshoes)

What does "actually" mean, what does it express in this context?

 

[emsr2d2]

It elaborates on the word "memory". Actually, it wasn't just a memory - it was, specifically, a tune.