Whether it’s love, support, or surrender, if something’s unconditional it’s absolute and not subject to any special terms or conditions: it’ll happen no matter what else happens.
Breaking apart the word unconditional can help you remember its meaning. Combine the prefix un-, meaning “not,” with conditional, meaning "dependent on something else," and you get an adjective for something that holds true without any conditions attached. The unconditional forgiveness you promised your brother means you forgive him no matter what. You’re not attaching any requirements — like you’ll only forgive him if he’s nicer to you or pays you money. You just forgive him — it's absolute.