A statement of
affirmation or agreement about a proposed activity. Used in place of "I could get behind that", or "sure". The 'yeah' can be omitted if in a hurry.
While
originally used exclusively about objects that are 'splayable', modern uses of the phrase do not require this property.
A similar phrase is "yeah, I *can* splay that", used to indicate belief or proficiency instead, i.e. "I'd buy it" or "I can do that".
It's also
theoretically possible to use "no, I couldn't/can't/won't splay that" to indicate distaste or refusal, but this is rarely heard.