What “disregarded” means
For state law, the LLC is a separate legal person: it owns bank accounts, enters contracts, and has limited liability. For US federal income tax, it has no separate identity — its income flows up to the single owner, who reports it on their own return.
The 5472 twist
Starting with tax year 2017, treasury regulations treat a disregarded entity that is wholly owned by a foreign person as a corporation for 5472 purposes only. The LLC still has no US income tax of its own, but it must file Form 5472 attached to a pro-forma 1120 if it has any reportable transactions with its foreign owner during the year.
This is the single most important rule to understand for a non-US founder with a US LLC: the LLC is “disregarded” for income tax, but it still has a 5472 information-filing obligation at the entity level.
See also: Form 5472, pro-forma 1120, single-member LLC.