What does John 14:25 mean?

Jesus once again notes He will not always be physically with the disciples (John 12:8, 35–36). He notes that He has told them certain things in advance, which serves several purposes. One major reason is to give these men confidence in the face of what is to come (John 16:32–33). Difficult experiences are often helped when we're told, in advance, exactly what to expect (John 14:29).

Another reason for Jesus' comment is expanded in the following verse (John 14:26). Many of the lessons taught by Christ could only be understood in the full context of His earthly ministry. The disciples can understand the broad strokes of His words now, but full realization won't be possible until they've seen exactly how His role as Messiah will be accomplished (John 2:18–22; 12:16; Luke 24:6–8). After seeing His death and resurrection, and obtaining the Holy Spirit, the disciples will finally have the full picture needed to preach the gospel to others.