en_tn/jhn/04/14.md

the water that I will give him will become a fountain of water in him

Here the word "fountain" is a metaphor for life-giving spring of water. Alternate translation: "the water that I will give him will become like a spring of water in him" (See: rc://en/ta/man/jit/figs-metaphor)

eternal life

Here "life" refers to the "spiritual life" that only God can give.