Late heavy bombardment (LHB) is an era of many impacts within the inner solar system, asteroids, hitting the Earth, Moon, Mars, Venus, and Mercury, within the span of time from 4.1 to 3.8 billion years ago. This is half a billion years after the formation of the solar system, and presumably after Earth got its water and its life began.
Evidence for it exists in lunar samples, from dating their most recent heating through radioactive dating. There are multiple theories regarding the source, one theory (the Nice model) being that planetary migration of the outer planets (Jupiter and those beyond) passed the planets through orbital resonances, increasing the orbits' eccentricity, leading to encounters with the asteroids of an at-the-time more populous asteroid belt, often throwing them inward, leading to many impacts with the rocky planets, and reducing the belt to its current population. This model can fit scenarios where Neptune falls into its current resonance with Pluto.