On the great day of Tabernacles, Jesus invites the thirsty to come to him and drink, then speaks of rivers of living water flowing from the believer. John immediately explains that Jesus is speaking about the Spirit.
The evangelist therefore does not leave the central meaning of the image entirely open to speculation.
Living water could ordinarily mean fresh or flowing water, but Israel's Scriptures also use water as an image of God's cleansing, renewal, and life-giving Spirit. Isaiah 44:3 places poured-out water and the pouring out of God's Spirit side by side. John says, 'as the scripture hath said,' although no single Old Testament verse exactly reproduces Jesus' wording; the saying appears to gather a cluster of scriptural water themes.
John also connects the promised reception of the Spirit with Jesus' future glorification.
The context shows that Jesus' invitation goes beyond relief of physical thirst or even the festival's water symbolism. He presents himself as the one through whom God's promised Spirit is received.
The image also turns recipients into channels of abundance: the life Jesus gives is pictured not as a private trickle but as rivers of living water.