What does Mark 11:25 mean?

This verse isn't found in Matthew's account, but Jesus does say something similar in the Sermon on the Mount: "For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses" (Matthew 6:14–15).

This is not to say that failing to forgive others or reconcile with those we've offended causes us to lose our salvation (Ephesians 1:13–14). Nor does it mean we can earn salvation by being forgiving (Titus 3:5). The point is the petty way in which we, as sinners, assume we have a strong relationship with God; meanwhile we often won't forgive comparatively minor offenses committed against us.

Jesus is pointing out a disparity in our thinking about forgiveness and relationships. God is the holy creator of the universe. We sin against Him every day. Every single sin makes us ineligible to be in His presence. But we still expect Him to forgive us and continue to give us what we ask. On the other hand, we get in petty disputes with other people on a regular basis. Thanks to pride or shame or apathy, we don't try to forgive, ask for forgiveness, and restore those relationships. God sacrificed His Son so that He can forgive us. While expecting God to forgive our heinous sins, we might not take five minutes to reconcile with someone no more a sinner than we are.

Jesus is comparing a dead fig tree with the spiritually dead Jewish traditions performed in the temple. The temple system is so corrupt that, like the tree, it no longer produces fruit, so it must be done away with. The temple is replaced by the body of every believer, which is indwelt with the Holy Spirit at the moment of belief in Jesus as our Lord and Savior (1 Corinthians 12:13). The sacrifices are replaced by Jesus' sacrifice on the cross. And rigid adherence to feasts, sacrifices, and ceremony are replaced with the willingness to forgive others.