Die Verabredung is a noun made from the verb verabreden.
The verb verabreden means to agree. You can boil it down to
reden – to talk
abreden, absprechen – to bargain, to negotiate
So, abreden and absprechen is alternating in talk. You may draw a line to abgeben which means to pass, and abfallen which means to come loose. The prefix ab- often means something got loose.
verabreden – to agree
So, verabreden is coming to a conclusion in abreden. Conclusion is a function ver- often carries.
In addition, there are multiple ways to make a noun from a verb:
der/die/das Verabredende (from Partizip I) — the person who makes an agreement
das Verabredete (from Partizip II) — the thing agreed upon
das Verabreden (from Infinitiv) — the action of making an agreement
die Verabredung (from Infinitiv) — the agreement
