Mediating God’s word means murmuring or speaking His word back to yourself, which will strengthen your mind because the God of truth is near you.
On the contrary, murmuring or speaking negative words to yourself or others will weaken your mind because the devil of lie is near you.
“Draw near to God and He will draw near to you” (James 4:8)
“…meditates on His law day and night…” (Psalm 1:2b)