Straight-through processing describes the plumbing: no human touches the order between click and execution. It says nothing on its own about whether the broker keeps the risk, since a firm can process straight through to its own book.
In practice STP is marketed as the alternative to a dealing-desk, with pricing coming from liquidity-aggregation and a small markup added to each side of the quote.
Example: the aggregated feed shows 1.08401 / 1.08413 and the broker adds 0.3 pips of markup per side, quoting 1.08371 / 1.08443. On one standard-lot round trip the markup is $6 and no commission is charged.
Related: ecn-broker, markup, no-dealing-desk, a-book