"mainlane" means: American transportation jargon for the primary through-traffic lanes on a highway or freeway, as distinct from on-ramps, off-ramps, service.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.