It basically means a change of mind and the context must determine what is involved in that change of mind. In passages where salvation is in view it is equivalent to believe or trust in and involves a change of mind about any form of self-trust in human works, good deeds, religious tradition, etc. followed by a trust in the finished work of Christ which alone has the power to save us. It means a turning from self-trust to trust in Christ. Believe and repent are never used together as if teaching two different requirements for salvation. Faith and repentance are like two sides of a coin.
Genuine faith includes repentance, and genuine repentance includes faith.
The repentance comes out of the recognition of one's sin, but the object of repentance is the person and work of Christ, or faith in Christ. A comparison of other passages clearly supports the fact that repentance often stands for faith in the person and work of Christ. If repentance, when used in connection with eternal salvation, is a separate or distinct requirement from faith in Christ, then John does not give the whole gospel. And if you can believe that, you can believe anything. It would have been most appropriate to use repent or repentance in the account of the Lord's conversation with Nicodemus.