Wenn ich das Büro abschließe, beginnt schon die zweite Besprechung.

Abschließen is a separable‐prefix verb (ab + schließen). In main clauses the prefix detaches and goes to the end:
• Ich schließe das Büro ab.
In subordinate clauses introduced by wenn, the entire verb (with prefix) goes to the final position:
• Wenn ich das Büro abschließe, …

German main clauses follow the “verb‐second” rule: the conjugated verb must be the second element. Here the first element of the main clause is the entire subordinate clause (Wenn ich das Büro abschließe), so beginnt comes right after it:

1st position: “Wenn ich das Büro abschließe,”
2nd position: beginnt
rest of the clause: “schon die zweite Besprechung.”

In German the Präsens often expresses future events when context makes the timing clear (here by wenn). Using present tense is more natural and common than the future tense (wird beginnen) for near or planned future actions.

Schon means “already” and conveys a sense of surprise or impatience that the second meeting is happening so soon.
• schon is colloquial and very common in spoken and written German.
• bereits is more formal but means the same: “Wenn ich das Büro abschließe, beginnt bereits die zweite Besprechung.”

A Besprechung is a formal discussion or meeting, often in a business context. It’s the standard German word for a planned gathering to talk over topics. While Germans also sometimes say Meeting (a loanword), Besprechung is purely German and equally—or more—common in everyday professional speech.

Besprechung is a feminine noun. In nominative singular it takes the article die. An ordinal number following a definite article uses the adjective (weak) ending -e. Hence:
die (fem. nom. sg.) + zweite → die zweite Besprechung