यतो यतो निश्चरति मनश्चञ्चलमस्थिरम्। ततस्ततो नियम्यैतदात्मन्येव वशं नयेत् ॥

yato yato niśhcharati manaśh chañchalam asthiram tatas tato niyamyaitad ātmanyeva vaśhaṁ nayet

As often as the wavering and unsteady mind goeth forth, so often reining it in, let him bring it under the control of the Self.

In plain words: Krishna says the mind is restless and wanders. Every time it wanders off, bring it back and place it under the control of the Self.