When playing a regular local audio stream the “Rec” button is disabled.
It automatically gets enabled when you start playing an Internet radio stream.
To start recording, simply hit the “Rec” button.
Please, note that Spider Player can record radio stations or any other content that streams in blocks. But if you are playing, for example, an audio file that resides on a remote server (e.g a podcast), that's not the case, so the record button won't lit up. To save such files, no special software needed, as you can download them by simply pasting a file's URL into a browser.
For more information, please read the Wikipedia article about Internet radio.
Related chapters.