Sola means a lot of things.
It can refer to the sky in the Engrish-‘sora’ sort of way. And obviously the connection between Sola and sky is quite clear, symbolic of what Matsuri has forgotten in her eternal journey of selflessness. The flashback to “LOL I remembered that it’s nice to have friends” bit is…well, very much about the sky that she has forgotten, and it’s a replay of when she recalls what it was like when she step into Yorito’s room. The sky is also a symbol of Yorito’s promise, which is fulfilled (in the lovingly SHOCKING TWIST fashion that makes AIR so good) when the broken-down film projector kicks in, and all that shizzat.
And speaking of AIR, the sky is full of air. Seeing that the scenarist behind Sola is the one behind Key’s successful hit games, the connection is, if anything, blatant. Sola is of AIR. Sola is also full of something else, especially episode 12…..anyways.
But I think Sola means being alone (Latin?). If you pay attention to the OP, in a way, it confirms that being alone is the best definition for the term as the show’s title. The series revolves around Matsuri’s loneliness issue and how it affects both her and the people connected to her. The lonely stroll under a rainy sky is a cute play-on-word.
And a cute play is so far what has transpired in the past 12 episodes. It was fun, it oozed with a good feeling; sort of like how Aria or Texhnolyze ooze with mood. I don’t think Sola can really disappoint with its final curtain draw now. A job well done so far, although not with its flaws.