In The Tempest, what is the meaning of Trinculo's line from Act 4 scene 1: 'we steal by line and level'?

Yes, Theatre Doc has got it but just to add something. The context is that in Shakespeare's day there was no formal setting of weight standards for selling goods, no Trading Standards to help you. Scales were frequently fixed with dodgy levels and measurements (lines) were cut short - you were, literally, given "short measure". Trinculo is admitting to a dishonest act. He is stealing just as all the traders steal by their short measure and fixed scales.