This is very good question.

As others already answered, “maru” means circle. But it is good question because there are many other meaning depending on the context. Maru is relatively old Japanese word (native Japanese word not like Chinese Japanese words introduced later.) Therefore there are many meanings which had derived from original meaning.

1. circle
   hinomaru = flag of japan
   Flag of Japan - Wikipedia
   akamaru, maru o kaku, marui kao
2. Correct answer
   maru desu = Your answer is correct
   gojumaru = excellent answer
3. Posfix for the name of ship.
   Sanadamaru, Tsushimamaru = name of ship
   Tsushima Maru - Wikipedia
4. 100%
   Maru de dameo = 100% incompetent boy.
   Marugoto = take 100%
   Marude Dameo - Wikipedia
5. Satisfaction
   Maru ku osameta = Resolved satisfactory
6. Suffix of number of castle
   honmaru = primary building in castle
   ninomaru = secondary building in castle
   Edo Castle - Wikipedia
7. Part of the given name for little boy
   Ushiwakamaru = name of Yoshitsune in youth.
   Minamoto no Yoshitsune - Wikipedia
   Goromaru
   Ayumu Goromaru - Wikipedia
8. Sometimes, it is used in name of place
   Simomaruko
   Shimomaruko Station - Wikipedia
9. Name of circler mark (Mon in Japanese)
   Tsurunomaru
   家紋-鶴（つる） - 家紋一覧／お仏壇のよねはら
10. Traditional name of unit (not used recently)
    1 Maru = 50 kin = 30kg
    1 Maru = 6 or 10 package of paper
11. Part of the name
    Marui (丸井) = name of department store
    https://www.0101.co.jp/
12. Zero
    0101 = maruimarui
    104 = hitomaruyon

And if you would like to cover all the meanings, please refer "広辞苑" which is one of the best Japanese dictionary. There are many other interesting meaning follows. It is worth explore.

『広辞苑　第七版』 - 岩波書店