dotspacemacs-frame-title-format "%t | %b"
`%t` is projectile-project-name, and `%b` is buffer name. So I will see something like "org | Tasks.org" as the frame title.
The default is `"%I@%S", which is invocation-name@system-name, e.g. emacs@neil-ThinkPad-T450s, which I don’t find that useful.