please sit down; please have a seat
Polite request used when offering a seat to someone. Often heard in formal settings or when welcoming guests.
Please, have a seat.
Please sit over here, sir/madam.
座る is the plain, neutral verb meaning 'to sit', while おかけ下さい is a polite honorific request form used to invite someone to sit.
Standard polite phrase meaning 'please sit down'. The kanji 下さい is commonly used in such requests.
Variant with the kanji 掛 for 'sit'. Less common but still seen.
Polite request form of the verb 掛ける (かける, 'to sit') with honorific prefix お and 下さい (ください, 'please').