"I play some guitar" means "I have a [very] limited talent for playing the guitar".
[Wordreference - USA]
''I play some guitar.''
Eu arranho (um pouco) no violão.
''I've been a guitarist for 35 years, but I play some drums
[eu arranho na bateria
], bass, and keyboards, as well, and I do some songwriting and home recording as well.'' [Musiciansfriend.com]
''I'm not much of an instrumentalist, but I play some piano