CSS gap property:

There is a new gap CSS property for multi-column, flexbox, and grid layouts that works in newer browsers now! (See Can I use link 1; link 2). It is shorthand for row-gap and column-gap.

#box {
  display: flex;
  gap: 10px;
}

CSS row-gap property:

The row-gap CSS property for both flexbox and grid layouts allows you to create a gap between rows.

#box {
   display: flex;
   row-gap: 10px;
}

CSS column-gap property:

The column-gap CSS property for multi-column, flexbox and grid layouts allows you to create a gap between columns.

#box {
  display: flex;
  column-gap: 10px;
}

Example:

#box {
  display: flex;
  flex-wrap: wrap;
  width: 200px;
  background-color: red;
  gap: 10px;
}
.item {
  background: gray;
  width: 50px;
  height: 50px;
  border: 1px black solid;
}

<div id='box'>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>