Use N space-delimited values, where N is the number of columns you want.
The values define the width of that column.
Width can be defined in px, %, em,
fr,
minmax(),
min-content,
max-content, and
auto.
As an alternative to space-delimited values, you can use
repeat().
repeat(3, 1fr) is equivalent to
1fr 1fr 1fr.