"that's why, therefore" — emphatic cause-effect. More colloquial than deshalb, often with "eben" ("darum eben"). Position 1 + inversion. Also a pronominal adverb meaning "around it".
Konjunktionaladverb kausal; umgangssprachlich mit Betonung.
Порядок слов
Hauptsatz 1 (Ursache)
Subj.
V = 2.
…
,
Ich
liebe
dich
,
Hauptsatz 2 mit darum
Konj.-Adv.
V = 2.
Subj.
…
darum
heirate
ich
dich.
A Konjunktionaladverb takes POSITION 1 of the second clause, so the verb lands in 2nd and the subject is pushed after the verb (inversion). This is the key difference from coordinating conjunctions (und, aber, denn), which stand outside position 1.