bring under control by force or authority
"suppress a nascent uprising"
"stamp down on littering"
"conquer one's desires"
take possession of by force, as after an invasion
"the invaders seized the land and property of the inhabitants"
"The army seized the town"
"The militia captured the castle"
To defeat in combat; to subjugate.
"The general managed to conquer the fortress after a fierce battle against the defending army."
In plain English: To conquer means to defeat an enemy and take control of their land so you become the ruler there.
"The army managed to conquer the enemy fortress after three days of fighting."
The word "conquer" entered Middle English from the Old French conquerre and ultimately derives from Late Latin conquaerere, which meant to knock or strike as well as to search for something. This borrowing replaced an earlier native Germanic term that had a similar meaning of overcoming someone through strength.