means In the figurative sense, the hidden conditions, obligations, or controlling connections attached to something — as in 'no strings attached' or 'pulling strings' to get a favor done.

from From Old English 'streng,' a cord or rope, with cousins across the Germanic languages (German 'Strang') and likely tied to the root meaning 'to draw tight.' The literal cord became a metaphor for control through the image of a puppet on its strings — and 'pulling strings' grew from that backstage picture of someone working the figures from above. 'No strings attached' is later commercial language, possibly echoing how cloth or goods were once tied with cords as a sign of conditions or claims on them.