。
We're running behind schedule.
Literal
Plan-than behind-[ongoing-ています contracted].
遅れてます is the colloquial drop of the い from 遅れています — extremely common in casual/neutral spoken Japanese. 遅れる ('to be late, to fall behind') pairs with より here for the comparative: 'later than [the plan].' Symmetric with the opposite expression 進んでいる ('is ahead, has advanced'), where Japanese uses state verbs where English would say 'ahead/behind.'