What is the mechanism of action of lisinopril, described in plain terms as inhibiting the narrowing of blood vessels?

Lisinopril is an ACE (angiotensin-converting enzyme) inhibitor: it blocks the enzyme responsible for converting angiotensin I into angiotensin II, a potent vasoconstrictor — so blocking this conversion prevents that vasoconstriction ('inhibits the narrowing of blood vessels') and reduces aldosterone-mediated sodium/water retention, lowering blood pressure.