Locks is a small, affectionate portrait of someone's hair. The speaker describes thick brown strands that stick up and out, then admits it looks "kind of silly." That casual, teasing tone suggests closeness: it's the way you'd talk about someone you're comfortable with, not someone you're trying to impress.
The shift comes in the last line. When the person runs their fingers through their hair, the speaker sees something unexpected: marble running through cracks in a wall. A moment ago the hair was just goofy, and now it's striking, even beautiful. Marble is smooth, cool and solid, while hair is soft and unruly, so the comparison makes an ordinary gesture feel almost like a discovery.
The mood is light, warm and observant. The speaker seems to be fond of the person, noticing how something silly can also be lovely, and letting the humor slowly turn into quiet admiration.