This is the 13351st most frequent Russian word.
мячи
"Мячи" means "balls" (plural form).
The word "мячи" is used in the nominative plural form here to indicate the presence of multiple balls on the field.
На поле лежат мячи.
There are balls lying on the field.
The word "мячи" is used as "мячами" in the instrumental plural form, describing tools or items children interact with.
Дети играют с мячами.
Children are playing with balls.
The word "мячи" is used as "мячей" in the genitive plural form to denote absence of balls in the possession.