To take a bath; to enter a bathtub to wash or warm oneself (daily action of bathing).
毎晩寝る前にお風呂に入るのが習慣だ。
I have a habit of taking a bath every night before bed.
仕事から帰ったらまずお風呂に入って汗を流したい。
When I get home from work I want to take a bath first and wash off the sweat.
温泉ではゆっくりお風呂に入るのが楽しみだ。
At a hot spring, I enjoy taking a long bath.
Colloquially can also mean to take a shower; in some contexts the phrase is used to refer to showering rather than bathing.
朝は時間がないから、お風呂に入らずサッとシャワーで済ませた（＝お風呂に入る代わりにシャワーを浴びた）。
I didn't have time in the morning, so I quickly took a shower instead of a bath.
夏は汗をかくから、帰宅後すぐお風呂に入る（＝シャワーを浴びる）ことが多い。
In summer I often shower right after coming home because I sweat a lot.
出張先のホテルでお風呂に入ってさっぱりした（シャワーを浴びた場合でも同様に言う）。
I felt refreshed after using the bath at the business hotel (the same is said even if it was a shower).